Evidence of a gap in student understanding has been well documented in chemistry: the typical student holds an abundance of misconceptions. The current expectation is that educational reform will foster greater student achievement via inquiry teaching within classrooms. Using assessments involving both conceptual and algorithmic knowledge of gas behaviour, gas laws, and phase changes in chemistry, this study aims to determine the relationship between inquiry teaching and student achievement. The topics were tested in the context of a unit from a reform-based curriculum that features inquiry, hands-on activities, and cooperative learning. This curriculum provides step-by-step guidance for each lesson in the teacher materials, in order to promote quality inquiry teaching. Two schools were involved in this study, with two high school chemistry teachers from each school. Each teacher participated in professional development and implemented this curriculum with sufficient training and guidance to develop reform methods. Student achievement was found to positively correlate with the use of inquiry teaching about the assessed concepts, regardless of teacher experience or school context.