One possible explanation for the occurrence of the frequent associations between red colobus monkeys and Diana monkeys we observed in Taï National Park, Ivory Coast, is that the two species meet by chance at commonly used food patches. An analysis of the overlap of diets showed, however, that foraging in the same trees could only explain a fraction of the total time in association. Therefore two functional explanations are considered: improvement of foraging efficiency and reduction of mortality due to predation. We did not find any evidence for an improved access to food sources. We conclude that, by default, the predation hypothesis provides the most plausible explanation.