This paper describes the analysis of teachers' journal reflections during an inquiry-based professional development program. As a part of their learning experience, participants reflected on what they learned and how they learned. Progress of subject matter and pedagogical content knowledge was assessed though surveys and pre- and posttests. We found that teachers have difficulties reflecting on their learning and posing meaningful questions. The teachers who could describe how they reasoned from evidence to understand a concept had the highest learning gains. In contrast those teachers who seldom or never described learning a concept by reasoning from evidence showed the smallest learning gains. This analysis suggests that learning to reflect on one's learning should be an integral part of teachers' professional development experiences.