Objectives The study of related species in contact zones can elucidate what factors mediate species coexistence and geographical distributions. We investigated niche overlap and group interactions of two gibbon species and their hybrids co‐occurring in a zone of overlap and hybridization. Methods The location, composition and behavior of white‐handed, pileated, and mixed‐species gibbon groups were studied by following them during 31 consecutive months in a relatively large part of the contact zone. Results Twenty groups of white‐handed gibbon were mapped followed by nine groups of pileated gibbons and five mixed‐species groups. White‐handed, pileated and mixed‐species groups had similar sizes and composition, ate a high proportion of fruits, shared a large number of species in their diets, and presented similar habitat preferences. Group home range sizes did not differ between species and overlapped little with neighboring groups irrespective of species, and intraspecific and interspecific encounter rates were similar. Discussion Ecological similarities support that competition between the gibbon species exists and takes the form of interspecific territoriality. However, we could not find any clear mechanism of niche partitioning favoring coexistence between species. Our findings suggest that the contact zone is unstable and is maintained by dispersal inward from groups of the parental species. The relatively low numbers of mixed‐species groups and hybrids found suggests a high degree of premating reproductive isolation, perhaps mediated by interspecific miscommunication. The existence of hybrids and backcrosses potentially undetectable from phenotypic characters alone raises the possibility of more widespread introgression than has been evident. Hence, while interspecific territoriality should reduce the rate of gene transfer, it would not necessarily present a barrier to introgression into contiguous populations of the opposite species.