There is a growing body of literature on the importance of proximity for innovation and other knowledge-related outcomes. We examine the impact of geographical, social, organisational, and cognitive proximity for a heterogeneous population, including people from academia, knowledge institutes, industry, and government. We analyse data on 1020 ego–alter relationships, derived from a survey among water professionals in the Netherlands. The use of survey data allows for more refined indicators of proximity and more diverse collaboration outcomes than those common in the literature. Social and cognitive proximity have a positive effect for all outcomes examined. Geographical and organisational proximity have a negative effect on hard (tangible) outcomes yet a weak positive (if any) effect on soft (intangible) outcomes. We do not find evidence for the suggestions in the conceptual literature that proximity follows an inverted U-curve where most outcomes are achieved in relations with some but not too much proximity.