Background: STEM educational reform encourages a transition from instructor-centered passive learning environments to student-centered, active learning environments. Instructors adopting these changes incorporate research-validated teaching practices that improve student learning. Professional development that trains faculty to implement instructional reforms plays a key role in supporting this transition. The most effective professional development experiences are those that not only help an instructor redesign a course, but that also result in a permanent realignment of the teaching beliefs of participating instructors. Effective professional development features authentic, rigorous experiences of sufficient duration. We investigated changes in the teaching beliefs of college faculty resulting from their participation in the Interdisciplinary Teaching about the Earth for a Sustainable Future (InTeGrate) project that guided them in the development of reformed instructional materials for introductory college science courses. A convergent parallel mixed methods design was employed using the Teacher Belief Interview, the Beliefs About Reformed Science Teaching and Learning survey, and participants' reflections on their experience to characterize pedagogical beliefs at different stages of their professional development. Results: Qualitative and quantitative data show a congruent change toward reformed pedagogical beliefs for the majority of participants. The majority of participants' Teacher Belief Interview (TBI) scores improved toward more student-centered pedagogical beliefs. Instructors who began with the most traditional pedagogical beliefs showed the greatest gains. Interview data and participants' reflections aligned with the characteristics of effective professional development. Merged results suggest that the most significant changes occurred in areas strongly influenced by situational classroom factors. Conclusions: The process of materials development employed in the InTeGrate project is comprised of rigorous, authentic, and collaborative experiences continued over a sufficient duration. Incorporating these characteristics in to a professional development program on a variety of scales can help promote the long-term adoption of reformed teaching strategies. Collaboration among geoscience professionals was one of the predominant drivers for change. Consequently, this research provides insight for the development of future professional development opportunities seeking to better prepare instructors to implement reformed instructional strategies in their classrooms.