Many primates are able to satisfy most of their water requirements from the liquid content of their food, but almost all need to visit external water sources during critical periods. The present study analyses the use of water sources in two free-ranging groups of buffy-headed marmosets (Caratinga and RBAR groups) over an annual cycle. At both sites, a large proportion of the days had no visits, and the visitation rate fell by approximately half in the wet season. Differences between sites appear to reflect both contrasts in habitat structure and the composition of the diet. Despite inhabiting a more humid environment, for example, the RBAR group visited water sources on a significantly larger proportion of days than the Caratinga group. While 94.2% of the records for the RBAR group involved bromeliads, this type of source was not recorded at Caratinga, where terrestrial sources were used relatively frequently. Seasonal variation indicates that arboreal sources are preferred over terrestrial ones at both sites, and that rivers are used as a last resort, when other sources are scarce. The use of terrestrial sources involves cautious and coordinated group behaviour, which presumably reflects the perceived predation risk. Overall, the relatively frequent use of watering sites in the RBAR group might be related to its highly fungivorous diet, in comparison with the more typical gummivorous diet of the Caratinga group, although the exact implications of this difference are not clear from the available data.