This paper discusses the nature of learner engagement with science content during small group discussions in which learners use their home languages. We observed that learners reverted to their home languages in small group discussions, yet very little is known about the dynamics of learner engagement when they use their home languages in classroom discussions in South Africa and elsewhere. We analysed transcripts of discussions by three small groups in a Grade 10 Chemistry class. Contrary to teachers' fears that learners may not engage meaningfully with science content when talking in their home languages, all three groups spent over 90% of discussion time on task. Learners made and supported claims, challenged each others' ideas and questioned each others' thinking. Although the levels of critique varied between the groups, there was evidence of negotiation of understandings of the concepts. We argue that use of learners' home languages for engagement with difficult concepts may be a legitimate resource for science teachers to create opportunities for learner conceptual understanding. Further research is needed to understand the dynamics of teacher and learner use of their languages in science lessons, the best teaching strategies to achieve this, how teacher educators may model these strategies without undermining the need by both parents and learners' for English language proficiency to access social goods.