This article reports research that is contextualised within reforms of secondary education in Hong Kong and the reintroduction of Liberal Studies, which jointly emphasise the need for a learning environment that facilitates the practice of group work and the development of critical thinking. A study is described that explores the relevance of group work for fostering critical thinking, looking in particular at the participation of teachers in small group activity. While attention is paid to the results of critical thinking tests and excerpts extracted from the students' dialogues, the research also seeks to highlight the role of the teacher in breaking stalemates in discussion with appropriate interventions. In sum, this article concludes by illustrating the importance of collaborative group work in the development of students' critical thinking skills. (Contains 3 figures and 3 tables.)