Inter-organizational collaboration is often seen as a means of increasing the potential for bringing about outcomes that none of the organizations could achieve alone. For example, funding agencies that provide grants for teacher professional development often encourage or require collaboration between institutions of higher education and schools. However, little research has been done on the complexities of working in these inter-organizational groups. In this qualitative study, we examined the experiences of an inter-organizational mathematics professional development planning group as it evolved over time and became a community of practice. Organizational psychology research was used as a lens to understand the challenges and changes that occurred for the group. Particular focus was given to the way inter-organizational effort can be affected by role ambiguity and can lead to collaborative advantage or inertia. The results of the study suggest ways in which professional development providers might increase their likelihood of achieving collaborative advantage. (Contains 1 table.)