Background: Argumentation, that is the coordination of evidence and reasons to support claims, is an important skill for democratic society, developing subject-specific literacies, and can be embedded in multiple school subjects. While argumentation has been extensively researched in science education, interdisciplinary argumentation is less explored, particularly between subjects where collaboration is not the norm, such as science and religious education (RE). Yet everyday issues often involve considering information from multiple sources, such as scientific information or ethical, moral, or religious perspectives. Purpose: The purpose of this study was to better understand students' abilities in argumentation within and across the school subjects of science and RE to inform research and practice of interdisciplinary argumentation. Sample: The participants of this study were 457 students, aged between 11 and 14 years, from 10 secondary schools in England. Following data cleaning, 394 student responses were analysed. Design and Methods: Students completed simultaneous written assessments for argumentation in three tasks which are situated within three different subject contexts: (1) science (2) RE, and (3) an interdisciplinary context which involved argumentation from science and RE. Results: In each of the three contexts, high proportions of students achieve all available marks for identifying claims and evidence. These proportions drop when constructing the link between claim and evidence (warrant) and constructing an evaluative argument. Higher performances were generally noted in the context of science and that students experience particular challenges in argumentation in the RE scenario. Conclusions: This study contributes to our understanding of the challenges and successes of students' argumentation within and across the subjects of science and RE. Implications for both research and practice are discussed.