Reform-based curriculum materials have been suggested as a mechanism to make inquiry-based instruction more prevalent in secondary science classrooms, specifically when accompanied by comprehensive professional development (Loucks-Horsley, Hewson, Love, & Stiles, [1998]; Powell & Anderson, [2002]). This research examines the implementation of a reform-based high school chemistry curriculum in a large, urban school district. We explicitly consider the role of the teachers' knowledge and beliefs in their implementation of the reform-based chemistry curriculum, as well as school level factors. Qualitative and quantitative data were collected in the form of beliefs interviews and classroom observations from 27 high school chemistry teachers. Analysis of the data revealed that implementation of the curriculum was strongly influenced by the teachers' beliefs about teaching and learning, and the presence of a supportive network at their school sites.