May 19, 2022

Research Article: “COCI, the OpenCitations Index of Crossref Open DOI-to-DOI Citations” (Preprint)

The following preprint has been submitted to the resource track of the 18th International Semantic Web Conference (ISWC).


COCI, the OpenCitations Index of Crossref Open DOI-to-DOI Citations


Ivan Heibi
University of Bologna

Silvio Peroni
University of Bologna

David Shotton
University of Oxford


via arXiv

In this paper, we present COCI, the OpenCitations Index of Crossref open DOI-to-DOI citations. COCI is the first open citation index created by OpenCitations, in which we have applied the concept of citations as first-class data entities, and it contains more than 445 million DOI-to-DOI citation links derived from the data available in Crossref. These citations are described in RDF by means of the new extended version of the OpenCitations Data Model (OCDM). We introduce the workflow we have developed for creating these data, and also show the additional services that facilitate the access to and querying of these data by means of different access points: a SPARQL endpoint, a REST API, bulk downloads, Web interfaces, and direct access to the citations via HTTP content negotiation. Finally, we present statistics regarding the use of COCI citation data, and we introduce several projects that have already started to use COCI data for different purposes.

Direct to Full Text (Preprint)
April 12, 2019.

See Also: Research Paper: “Crowdsourcing Open Citations with CROCI: An Analysis of the Current Status of Open Citations, and a Proposal” (Preprint)
Posted on February 8, 2019.

See Also: An Introduction to the Crossref Open Citation Index (COCI)
Posted on July 12, 2019.

Direct to Crossref Open Citation Index (COCI)

About Gary Price

Gary Price ( is a librarian, writer, consultant, and frequent conference speaker based in the Washington D.C. metro area. Before launching INFOdocket, Price and Shirl Kennedy were the founders and senior editors at ResourceShelf and DocuTicker for 10 years. From 2006-2009 he was Director of Online Information Services at, and is currently a contributing editor at Search Engine Land.