Liberal studies is a new subject in the new senior secondary curriculum in Hong Kong. This study attempted to identify factors that shape liberal studies student teachers' decisions in lesson planning. Based on interview data and lesson plans developed by the student teachers, this article maintains that four factors interacted with the subject features of liberal studies to shape the student teachers' planning decisions, namely, teacher knowledge, curriculum requirements, materials and resources, and interaction with equal and more capable partners. This paper further looks into how teacher knowledge expresses itself at different stages of lesson planning. The findings suggest that to facilitate enquiry learning in students, liberal studies teachers need to adopt an enquiry stance in lesson planning. This paper argues that the limitations of liberal studies student teachers' content knowledge need to be addressed in teacher preparation programmes and suggests teacher networking as one way to strengthen novice liberal studies teachers' content preparation. The implications for teacher education are discussed. (Contains 1 table.)