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(000) surface formed via dissociation of water on oxygen vacancy sites; (2) partial dissociation of water on the mixed-terminated ZnO(100) surface yielding coexistent H₂O (∼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(100) surface; (4) interaction of water with defects forming hydroxyl (or O–H⋯O) species (3564 and 3448 cm⁻¹).