A mixed methods research approach was used to investigate the impact of a geosciences research institute upon 62 science teachers' knowledge, beliefs, and teaching practices related to teaching the geosciences. Pre- and postinstitute quantitative and qualitative assessments revealed mixed results. Results of a quantitative measure found a statistically significant increase in the teachers' self-reported overall knowledge of the geosciences, knowledge of watersheds, and knowledge of brownfield sites. However, a more specific quantitative instrument that measured watershed knowledge indicated no change except in the teachers' awareness that the water quality in different towns is related. In addition, their science teaching self-efficacy, which was used as a quantitative measure of the teachers' beliefs about teaching science in general, showed a statistically significant decrease. Focus group discussions provided an indication as to why the teachers' self-efficacy decreased, as teachers described numerous barriers that limited their capacity to engage their students in field-based research projects even while noting the benefits of such an experience. Although the study was limited in the ability to measure direct impact on instructional practices, the teachers' perception of their confidence in their capability to use inquiry-based instruction in the classroom showed a statistically significant increase. The discrepant results provide useful information about the complex impact of a professional development experience upon teachers, while identifying possible limitations of the experience and of the assessment measures, as well as providing recommendations for future research.