The interaction of water with ZnO nanoparticles has been studied by means of diffuse reflectance infrared spectroscopy (DRIFTS) and ultra-high vacuum FTIR spectroscopy (UHV-FTIRS). Exposing clean ZnO powder to water at 323 K leads to both molecular and dissociative adsorption of H2O forming a number of hydroxyl species. All the OH bands are clearly identified by the adsorption of D2O showing the expected isotopic shifts. According to the vibrational and thermal stability data obtained from single crystal surfaces, the OH species observed on ZnO nanoparticles are identified as follows: (1) OH group (3620 cm(-1)) on the polar O-ZnO(0001[combining macron]) surface formed via dissociation of water on oxygen vacancy sites; (2) partial dissociation of water on the mixed-terminated ZnO(101[combining macron]0) surface yielding coexistent H2O ( approximately 3150 and 3687 cm(-1)) and OH species (3672 cm(-1)), where the molecularly adsorbed H2O is further identified by the characteristic scissoring mode at 1617 cm(-1); (3) isolated OH species (3639 and 3656 cm(-1)) formed on the mixed-terminated ZnO(101[combining macron]0) surface; (4) interaction of water with defects forming hydroxyl (or O-HO) species (3564 and 3448 cm(-1)).