Previous quantitative studies suggest that the burden researchers who use English as an additional language perceive when writing research articles (RAs) for publication in English (as L2) is 24% greater than the burden they perceive when they write RAs for publication in their L1. It remains unclear precisely which aspects of research article (RA) writing in English present these writers with the greatest challenge and just why they perceive this increase in difficulty. A structured questionnaire comprising thirty-seven questions about researchers’ publication experiences in scientific journals in English and in Spanish was designed and sent out to all (n = 8,794) Spanish postdoctoral researchers at one research-only institution and four universities in Spain, yielding responses from 1,717 researchers. Our first results show that the discussion is the section that is perceived as more difficult to write for English-medium journals, across the four broad knowledge areas in a way that cannot be fully explained by their lower level of proficiency in English (as L2). This article proposes the rhetorical transfer hypothesis as a possible explanation for their additional difficulty. Our results also reveal that their increased perceived difficulty writing RA discussions in English (as L2) does not decrease noticeably until Spanish researchers report high or very high levels of proficiency in English (as L2) for academic or general purposes or have published on average at least 37 RAs as corresponding author in English-medium journals over the last ten years. Implications for English for Academic Purposes (EAP) research and pedagogy are discussed.