Nearly two dozen journals from two of the fastest growing open-access publishers, including one of the world’s largest journals by volume, will no longer receive a key scholarly imprimatur. On 20 March, the Web of Science database said it delisted the journals along with dozens of others, stripping them of an impact factor, the citation-based measure of quality that, although controversial, carries weight with authors and institutions. The move highlights continuing debate about a business model marked by high volumes of articles, ostensibly chosen for scientific soundness rather than novelty, and the practice by some open-access publishers of recruiting large numbers of articles for guest-edited special issues.

The Web of Science Master Journal List, run by the analytics company Clarivate, lists journals based on 24 measures of quality, including effective peer review and adherence to ethical publishing practices, and periodically checks that listed journals meet the standards. Clarivate calculates impact factors for a select subset of journals on the list. The company expanded quality checks this year because of “increasing threats to the integrity of the scholarly record,” Web of Science’s Editor-in-Chief Nandita Quaderi says. The company removed 50 journals from the list, an unusually large number for a single year, and Clarivate said it is continuing to review 450 more, assisted by an artificial intelligence (AI) tool.

“My expectation is that this initial delisting … is only the tip of the iceberg,” says Rob Johnson, managing director of Research Consulting, a firm that advises science publishers and funders.

Journals losing their Web of Science impact factors—the average number of citations per article over 2 years—is bad news for their authors because the metric is widely used in hiring, tenure, and promotion decisions as a proxy for quality, despite criticism that impact factors are methodologically flawed. The affected journals will also likely have difficulty attracting future authors.

Clarivate initially did not name any of the delisted journals or provide specific reasons. But it confirmed to Science the identities of 19 Hindawi journals and two MDPI titles after reports circulated about their removals. The MDPI journals include the International Journal of Environmental Research and Public Health, which published about 17,000 articles last year. In 2022, it had a Web of Science journal impact factor of 4.614, in the top half of all journals in the field of public health.

In a statement, MDPI said Clarivate removed the MDPI journals based on Web of Science’s “content relevance” criterion. MDPI’s practice of publishing large numbers of special issues is likely at the heart of the concern, outsiders say. “[Clarivate’s] announcement suggests we are approaching the high-water mark for the use of special issues as a growth model,” Johnson says.

Articles in special issues account for most of the meteoric growth of MDPI since it was founded in 2010. Formerly called the Multi-disciplinary Digital Publishing Institute, it has since become the largest publisher of open-access papers and the fourth-largest publisher of scholarly papers overall, producing some 400 journals. Publishers of open-access papers typically charge authors a fee and make them immediately free to read when published. In 2022, nearly 100 MDPI journals that have impact factors published more than 17,000 special issues, containing 187,000 articles, according to an unpublished analysis by Paolo Crosetto, an economist at France’s National Research Institute for Agriculture, Food, and the Environment, and Pablo Gómez Barreiro of the Royal Botanic Gardens, Kew. (Their analysis extends a 2021 version.) Some MDPI titles published four special issues a day.

Skeptics worry the practice is especially vulnerable to manipulation by guest editors who lack expertise, have conflicts of interest, or accept fabricated manuscripts produced by paper mills. “I have no proof that they did anything wrong,” Crosetto says, adding that guest editor practices appear to vary. “But it stands to reason that trust [in the product’s quality] is hard when you leave this guest editing to anyone.”

Carlos Peixeira Marques of the University of Trás-os-Montes and Alto Douro, for example, says MDPI sent him multiple invitations to serve as a guest editor, in agriculture, animal science, and engineering—but never in his field of business and tourism. “The absolutely insane number of [MDPI] special issues has made it impossible to guarantee minimum peer-review standards,” he says.

The speed with which MDPI’s special issue manuscripts are reviewed and published is also a concern, Crosetto says. In 2022, MDPI’s median time from submission to acceptance was 37 days, well below the 200 days at the PLOS family of journals, another large, open-access publisher he examined for comparison. For about one in three papers, MDPI’s turnaround was 1 month or less. Considering the time it can take to recruit reviewers and revise manuscripts, “this looks just impossible,” he says.

Attempts by Science to reach MDPI for comment were unsuccessful. But in past statements, the company and supporters have said fast peer review, done properly, can allow authors to share results with colleagues more quickly than via traditional journals. Serving as a guest editor can help junior researchers develop editing skills and network with colleagues. And the company says it provides an outlet for technically sound papers, including ones with negative results, that might be rejected by more selective journals.

Special issues have been also been problematic for Hindawi. Its owner, scholarly publisher Wiley, announced on 9 March that it suspended publishing special issues in Hindawi journals from mid-October 2022 to mid-January after it identified “compromised articles.” The special issues were targeted by paper mills and “bad actors” who fabricated content, says Jay Flynn, Wiley’s executive vice president.

In response, Wiley added staffing and increased editorial controls, pairing AI-based screening with manual checks, he said. This led to 500 retractions in Hindawi journals, with an estimated 1200 more to come. Hindawi reported a loss of $9 million in revenue from the pause in special issues for the quarter ending in January. The 19 Hindawi journals delisted by Clarivate represent about one-third of its journals that had been listed in Web of Science.

MDPI isn’t the only publisher expanding the number of articles that authors paid to publish. In 2022, 55 open-access biomedical journals each carried more than 2000 full articles, up from 11 in 2015. The 300,000 articles from these “megajournals” in 2022 was one-quarter of all literature in that field that year, according to a commentary published on 20 March in JAMA by John Ioannidis of Stanford University and colleagues.

“Megajournals may perpetuate and accentuate an already dysfunctional system of scientific evaluation and publication,” they write. The pay-for-publication model creates an incentive for authors trying to meet institutions’ quotas for publications, and “megajournals may drain an already strained pool of reviewers from traditional journals.” Ioannidis calls for more research comparing the quality of peer review in megajournals and traditional ones, and he suggests institutions and funders reward researchers for studies that are transparent and rigorous.