MgO thin films were deposited with r.f. magnetron sputtering method. Deposition parameters such as working pressure and r.f. power were found to influence properties of deposited films. As working pressure increased, crystallinity deteriorated while rms surface roughness decreased and film density increased. In this case, grain size did not change much compared with the case of r.f. power. On the other hand, as r.f. power increased, both grain size and rms surface roughness increased while film density decreased. An increase of r.f. power improved crystallinity a little and induced (1 0 0) preferred orientation. But the amount of variation was small compared with the case of pressure. Sputtered MgO thin films were hydrated by exposing in the humid ambience with 80% relative humidity at room temperature. Hydration reaction produced clusters, Mg(OH)(2), on the surface of the films. Hydration also occurred in the inner part of MgO thin film. The entire hydrated region was observed to be approximately 2 nm thick. Therefore, the factors that influence hydration seem to be density and grain size of MgO films. MgO film with high density can restrict inter-diffusion of Mg atom and H2O, which is an important process for hydration. A reduction of grain size increased the area of grain boundary, which provided nucleation sites of hydration reaction as well as the short path of inter-diffusion of Mg atoms and H2O molecules. From the results, the relation between deposition condition and hydration characteristics of MgO thin films can be established. (C) 2003 Elsevier Science B.V. All rights reserved.