In content and language integrated learning (CLIL) classrooms, it is assumed that non-language content subjects provide more authentic communicative contexts for students to learn a foreign/second/additional language (L2). However, learning abstract concepts and academic language in an L2 simultaneously is also challenging for CLIL students. It is thus important for CLIL teachers to unpack and repack both abstract concepts and academic discourse for the students. ‘Semantic waves’, which model classroom practices of both unpacking and repacking, is arguably a key to understanding cumulative knowledge-building. Applying the concepts of semantic profiles and semantic waves, this paper analyses the classroom discourse of two CLIL science lessons in Hong Kong. In one lesson, the semantic profile mainly consists of downward shifts. The teacher adopted various useful strategies to unpack science concepts, especially with multimodalities, everyday L2 and students’ L1 resources. Yet, there was limited repacking. In contrast, some repacking was observed in another lesson, where the teacher provided explicit instruction on academic language and guided students through academic writing tasks. A semantic wave can thus be observed there. These findings on strategies for unpacking and repacking provide significant insights into knowledge building in CLIL contexts, and may hence illuminate CLIL pedagogical practices. © 2020 Informa UK Limited.