Research on Content and Language Integrated Learning (CLIL) programmes has shown that teachers find it difficult to integrate content and language in their teaching, and that this may be due to a lack of a specific type of teacher language awareness (TLA) for this type of teaching. This study explores how TLA for CLIL is manifested in metatalk in the context of a collaborative professional development activity, which took place in a bilingual secondary school in Madrid (Spain). Content and language teachers in pairs designed content and language integrated instructional sequences, implemented them, and assessed the students' output. Four sessions in which teachers reflected on the activity, in collaboration with researchers, were audio recorded, transcribed, and analyzed. The data were analyzed using a social realist framework for investigating knowledge-building practices, Legitimation Code Theory (LCT). The analysis showed how content and language teachers refer to three types of knowledge (content, language, and language and content integration) in terms of 'what' is known (conceptual frameworks) and 'how' it is approached (following specific methods). The results confirm the importance of developing CLIL teachers' language awareness through on-site collaboration which is closely linked to their normal teaching activities.