We observed UV-stimulated hydroxylation at the surface of ZnO crystals. Reactive defective sites were initially formed in the surface layer via photoreduction induced with energetic photons above the band gap of ZnO. Hydroxyl groups were produced by a chemical reaction of the photoinduced defective sites with water molecules in the atmosphere. Two types of hydroxyl groups were found at the irradiated surface because two kinds of defective sites were induced with the UV illumination. (C) 2003 Elsevier B.V. All rights reserved.