Al2O3 capping layer was deposited by DC reactive sputtering method on MgO protective layer deposited by RF magnetron sputtering method. Thickness of capping layer, Al2O3, was varied from 1 to 15 nm. Deposited MgO thin films were hydrated in the ambience of 80% humidity, and at room temperature. Surface morphology and rms roughness were observed by SEM and AMF, respectively. Chemical shift of electron binding energy was observed by XPS. And the composition and concentration of hydrogen were observed by RBS and ERD. From these analyses, it was found that Mg atoms diffused into Al2O3 layer and reacted with moisture at surface, forming Mg(OH)(2) phase during hydration reaction. At the thin capping layer region (1-5 nm thickness), as thickness of Al2O3 increased, total amount of hydration decreased. But, beyond 5 nm thickness, the tendency became reverse. Although Al2O3-capped MgO thin film was hydrated to some extent, total amount of hydration is much less than uncapped MgO single layer. Therefore, it can be concluded that Al2O3 might be suitable for protecting MgO layer against hydration. (C) 2003 Elsevier Science B.V. All rights reserved.