While research on CLIL suggests positive impacts on lexical proficiency and on spoken language, the crucial question of the effect of CLIL on advanced learners, both in terms of subject-specific language (SSL) proficiency and content knowledge, has received less attention. We argue that the ability to negotiate a factual position appropriately is a key element of SSL, relating to both content and to foreign language ability. As a theoretical framework for understanding these negotiations of generally opposing standpoints, we use argumentation theory. The study was conducted in upper-secondary CLIL classes on European economics and politics in Austria. From the data set of 16 hours of video-recording, comprising teacher-whole class interactions, group work, and role plays, episodes of argumentation were extracted and analysed. Two types of argumentation patterns emerge, with one focused on the joint construction and learning of new SSL and content knowledge, and the other on the enactment of such knowledge in interaction. Students' engagement in these argumentations gives clear evidence of their SSL proficiency in prepared and unprepared oral production. As the type of classroom event has an effect on the specific argumentation patterns, the need for exposing students to a diversity of educational practice is underlined.