Initiative for Open Citations

The Initiative for Open Citations I4OC is a collaboration between scholarly publishers, researchers, and other interested parties to promote the unrestricted availability of scholarly citation data.


Citations are the links that knit together our scientific and cultural knowledge. They are primary data that provide both provenance and an explanation for how we know facts. They allow us to attribute and credit scientific contributions, and they enable the evaluation of research and its impacts. In sum, citations are the most important vehicle for the discovery, dissemination, and evaluation of all scholarly knowledge.

As the number of scholarly publications is estimated to double every nine years, citations – and the computational systems that track them – enable researchers and the public to keep abreast of significant developments in any given field. For this to be possible, it is essential to have unrestricted access to bibliographic and citation data in machine-readable form.

The present scholarly communication system inadequately exposes the knowledge networks that already exist within our literature. Citation data are not usually freely available to access, they are often subject to inconsistent, hard-to-parse licenses, and they are usually not machine-readable.

An initiative to open up citation data

The aim of this initiative is to promote the availability of data on citations that are structured, separable, and open.

Structured means the data representing each publication and each citation instance are expressed in common, machine-readable formats, and that these data can be accessed programmatically. Separable means the citation instances can be accessed and analyzed without the need to access the source bibliographic products (such as journal articles and books) in which the citations are created. Open means the data are freely accessible and reusable.

Key benefits of achieving this aim include:

Reference distribution

Many publishers currently deposit reference lists from their journal articles to Crossref as part of their participation in Crossref’s Cited-by service. To open their references, along with the other bibliographic metadata that publishers send to Crossref, publishers need to turn on reference distribution for all of the Digital Object Identifier (DOI) prefixes they manage. This step allows references within the Crossref members’ articles to be distributed without restriction through all of Crossref's Metadata Delivery services, including the REST API and bulk metadata dumps, to any interested party. See below for additional information on reference distribution and on how to participate in Crossref’s Cited-by service.

Relationship with other initiatives

I4OC is part of a broader movement for promoting openness of bibliographic metadata. In particular, I4OC has a close relationship with the Initiative for Open Abstracts (I4OA), a sister initiative of I4OC aimed at promoting openness of abstracts of scholarly publications. I4OC and I4OA are managed by different teams, but these teams consist partly of the same individuals. While we hope that scholarly publishers will make a general commitment to openness of bibliographic metadata and will support both initiatives, it is possible for a publisher to support I4OC without supporting I4OA, or vice versa.

Participating publishers

Before I4OC started, publishers releasing references in the open accounted for just 1% of citation metadata collected annually by Crossref. Following discussions over the past months, several subscription-access and open-access publishers have recently made the decision to release reference list metadata publicly. These include: American Geophysical Union, Association for Computing Machinery, BMJ, Cambridge University Press, Cold Spring Harbor Laboratory Press, EMBO Press, Royal Society of Chemistry, SAGE Publishing, Springer Nature, Taylor & Francis, and Wiley. These publishers join other publishers who have been opening their references through Crossref for some time.

Starting from 3rd June 2022 all the members of Crossref cannot limit the distribution of their references. All the members that previously had limited or closed references have now been set to open. This means that all the currently deposited references in Crossref are now treated as open metadata. You can read the dedicated blog post explaining the new membership terms of Crossref.

The following list includes the names of all those publishers who, as of August 16, 2022, have submitted to Crossref the references from at least one publication bearing a Crossref DOI. I4OC updates this list, which presently includes about 5,343 names, every two months.

There are more than 13,000 additional publishers who are depositing publication metadata with Crossref, but who are failing to submit references with the other publication metadata. I4OC appeals to these publishers to start depositing their publication references, and other rich metadata including publication abstracts, ORCID author identifiers, and funder information, along with the basic metadata they already submit to Crossref.

How many citations are open today?


As of August 2022, the fraction of publications with open references has grown from 1% to 100% out of 61 million articles with references deposited with Crossref.

We encourage all other scholarly publishers to follow the example of these trail-blazing publishers by making their reference metadata publicly available. Please contact Crossref Support ([email protected]) for more information, or to let them know that you are ready to open up your reference metadata now. See also our list of responses to frequently asked questions.

Building on open citations

Several organizations and projects have expressed support for the Initiative for Open Citations and interest in building on and promoting the availability of open citation data. I4OC will keep a list of these projects, and we encourage all other interested parties to make contact with us.

sloan logo
allen logo
altmetric logo
arl logo
authorea logo
Bill and Melinda Gates Foundation Logo
british library logo
cdl logo
cos logo
cwts logo
citeseerx logo
coko foundation logo
coar logo
contentmine logo
core logo
data carpentry logo
dataverse logo
dblp logo
dryad logo
disi logo
eigenfactor logo
europe pmc logo
figshare logo
harvard library osc logo logo
impactstory logo
issi logo
internet archive logo
jisc logo
knowledge lab logo
liber logo
mpdl logo
microsoft research logo
mozilla logo
oaspa logo
okfn logo
openaire logo
orfg logo
overleaf logo
jupyter logo
ropensci logo
scienceopen logo
science sandbox logo
sparc logo
wellcome trust logo
wikiedu logo
wmde logo
wmuk logo
zotero logo

Founding organizations

The idea of creating an advocacy group to promote the availability of citation data gained momentum at the 8th Conference on Open Access Scholarly Publishing (COASP 2016), in response to a report that only a tiny minority of the almost 1,000 publishers depositing references with Crossref at that time were making this data publicly available.

Six organizations collaborated to form I4OC:


OpenCitations logo
Wikimedia Foundation logo
Public Library of Science logo
eLife logo
datacite logo
ccat logo

The creation of I4OC was spearheaded by: Jonathan Dugan, Martin Fenner, Jan Gerlach, Catriona MacCallum, Daniel Mietchen, Cameron Neylon, Mark Patterson, Michelle Paulson, Silvio Peroni, David Shotton, and Dario Taraborelli.

Frequently asked questions

If not already a participant in Cited-by, a Crossref member can register for this service free-of-charge. Having done so, there is nothing further the publisher needs to do to ‘open’ its reference data, other than to give its consent to Crossref, since participation in Cited-by alone does not automatically make these references available via Crossref's standard APIs.

We encourage all publishers to make their reference metadata publicly available. If you are already submitting article metadata to Crossref as a participant in their Cited-by service, opening them can be achieved in a matter of days. Publishers can easily and freely achieve this:

  • either by contacting Crossref support directly by e-mail, asking them to turn on reference distribution for all of the relevant DOI prefixes;

  • or by themselves setting the <reference_distribution_opt> metadata element to "any" for each DOI deposit for which they want to make references openly available.

Once made open, the references for individual scholarly publications may be accessed within a few days through the Crossref REST API. For information on how to use the interface, see their REST API guide (example query:

Open citations are also available from the OpenCitations Corpus, a database created to house scholarly citations, that is progressively and systematically harvesting citation data from Crossref and other sources. An advantage of accessing citation data from the OpenCitations Corpus is that they are available in standards-compliant machine-readable RDF format, and include information about both incoming and outgoing citations of bibliographic resources (published articles and books).

It can include both, depending on what references the publisher has deposited. All DOIs under a prefix set for open reference distribution will have open references through Crossref for past, present, and future publications.

Crossref exposes article and reference metadata without a license, since it regards these as raw facts that cannot be licensed.

The structured citation metadata within the OpenCitations Corpus are published under a CC0 public domain dedication, to make it explicitly clear that these data are open.

No. Although Open Access articles may be open and freely available to read on the publisher’s website, their references are not separate, and are not necessarily structured or accessible programmatically. Additionally, although their reference metadata may be submitted to Crossref, Crossref historically set the default for references to “closed,” with a manual opt-in being required for public references. Many publisher members have not been aware that they could simply instruct Crossref to make references open, and, as a neutral party, Crossref has not promoted the public reference option. All publishers therefore have to opt in to open distribution of references via Crossref.

For Crossref metadata, their REST API reveals how many and which publishers have opened references. Any system or tool (or a JSON viewer) can be pointed to this query: to show the count and the list of publishers with "public-references": true.

To query a specific publisher’s status, use, for example, then find the tag for public-references. In some cases it will be set to false.