Although public research organizations (PROs) are potentially valuable collaboration partners for firms in the development of innovations, most firms find it difficult to develop and sustain fruitful collaborations with PROs. Proximity dimensions, such as geographical, cognitive, organizational, and social proximity, are important facilitators of inter-organizational collaboration. Nevertheless, our understanding of the interaction between and evolution of different proximity dimensions over time is limited. Based on a longitudinal study of 15 successful innovation projects involving firms and PROs as collaboration partners, we find that different proximity dimensions are important for the establishment of new collaborations, depending on a firm's characteristics. While engineering-based firms tend to rely on geographical and social proximity to PROs, science-based firms rely more heavily on cognitive and organizational proximity. Moreover, we observe that firms with initial social and geographical proximity to PROs can sustain and expand their collaborations by developing cognitive and organizational proximity over time.