A Quantitative Portrait of Diamond Open Access in the European Union

Diamond Open Access (DOA) -- where neither authors nor readers bear direct costs -- is attracting growing institutional attention as an alternative framework for equitable scholarly communication. Despite the recent efforts to formalise its defining criteria and develop a dedicated discovery infrastructure, a systematic comparative analysis of DOA at the European regional scale remains lacking.This paper addresses that gap by providing a quantitative analysis of DOA publishing across EU countries, leveraging the Diamond Discovery Hub (DDH) as a curated source of DOA journals and the OpenAIRE Graph -- a large-scale open Scientific Knowledge Graph -- for article metadata and citation data from 2000 to 2024.Our methodology distinguishes between an author-side perspective (i.e., papers produced by researchers affiliated with a given country) and a journal-side perspective (i.e., papers published in journals hosted by that country), enabling characterisation of each country as a research producer, a publishing hub, or both.Results reveal consistent growth in DOA journal counts, article output, and citation accumulation across Europe. Significant national variation emerges: Croatia and Slovenia lead in DOA share relative to overall output; Spain and Poland dominate in absolute volume; and France stands out for citation impact and international journal attractiveness. Germany presents a structural asymmetry -- hosting internationally visible DOA journals while domestic researchers engage comparatively little with the model.Notably, the Social Sciences, Arts, and Humanities predominate across all countries.These findings advance the scientometric understanding of open scholarly communication and highlight how shared digital infrastructures -- such as open Scientific Knowledge Graphs and journal registries -- enable large-scale monitoring of emerging publishing models.