With the announcement of several new diamond open access (OA) related initiatives and the creation of the Global Summit on Diamond Open Access, diamond OA is now at the forefront of the OA movement. However, while working on our recent Quantitative Science Studies publication and data sets, we noticed that temporarily waiving article processing charges (APCs) was a commonly used strategy by big publishers for some of their journals. In the absence of an index of diamond journals, most studies have operationalized the identification of diamond journals as a subset of gold journals that do not charge an APC. While this is a pragmatic approach, we fear that it could undermine the value of the research in understanding what we believe is more commonly understood by diamond OA. This letter discusses the need for bibliometric research to apply more nuance in how it operationalizes diamond OA beyond the absence of APCs. We call on the publishing sector to be more transparent in the costs of publishing. Ultimately, we argue that transparency and a long-term commitment to no-APC publishing are necessary for diamond OA to succeed, and that the research community needs to apply this standard when seeking to understand the model.

We aim to estimate the total amount of article processing charges (APCs) paid to publish open access (OA) in journals controlled by the five large commercial publishers (Elsevier, Sage, Springer Nature, Taylor & Francis, and Wiley) between 2015 and 2018. Using publication data from WoS, OA status from Unpaywall, and annual APC prices from open data sets and historical fees retrieved via the Internet Archive Wayback Machine, we estimate that globally authors paid $1.06 billion in publication fees to these publishers from 2015–2018. Revenue from gold OA amounted to $612.5 million, and $448.3 million was obtained for publishing OA in hybrid journals. Among the five publishers, Springer Nature made the most revenue from OA ($589.7 million), followed by Elsevier ($221.4 million), Wiley ($114.3 million), Taylor & Francis ($76.8 million), and Sage ($31.6 million). With Elsevier and Wiley making most of their APC revenue from hybrid fees and others focusing on gold, different OA strategies could be observed between publishers.