CO oxidation reactivity of bare and TiO2-coated nanoparticles consisting of both NiO and Ni(OH)(2) surfaces was studied. For the deposition of TiO2, atomic layer deposition was used, and formation of three-dimensional domains of TiO2 on NiOeNi(OH)(2) could be identified. Based on the data of X-ray Photoelectron Spectroscopy, we suggest that upon TiO2 deposition only Ni(OH)(2) was remained on the surface, whereas NiO surface disappeared. Both CO adsorption and CO oxidation took place on NiOeNi(OH)(2) surfaces under our experimental conditions. CO adsorption was almost completely suppressed after TiO2 deposition, whereas CO oxidation activity was maintained to large extent. It is proposed that bare NiO cannot be active for CO oxidation, and can only uptake CO under our experimental condition, whereas hydroxylated surface of NiO can be active for CO oxidation. (C) 2011 Elsevier B. V. All rights reserved.