Despite the tremendous efforts which have been made over the last few years, inquiry-based learning (IBL) has not become an established practice in science education yet. Even if teachers consider IBL an important and valuable approach, they still refrain from implementing it in their own science classes. Research findings suggest that the teachers' beliefs and attitudes have a crucial influence on their classroom practice. Only if they are compatible with the approach of IBL, the teachers will be willing to implement IBL in their own classes. This article demonstrates how the beliefs and attitudes of three Austrian chemistry teachers changed over the period of a 9-monthly professional development program (PDP). In order to ascertain their beliefs and attitudes regarding IBL previous to and after the PDP, we conducted a group discussion and three individual interviews. The statements from both dates were analyzed using a qualitative content analysis and indicate substantial changes concerning the teachers' beliefs about the efficacy, capability and practicability of IBL. After participating in the PDP, the teachers took a more positive attitude regarding both IBL in general and its implementation. Considering the findings from this study, we formulate implications for designing PDP at the end.