We studied changes in magnesium oxide upon rehydration under operating conditions similar to those used in the subsequent impregnation of a metal salt on its surface. The ensuing transformations were monitored by using the XRD, TPR, and H-1 MAS NMR techniques. On the basis of the results, magnesium oxide (periclase phase) is rehydrated to magnesium hydroxide (brucite phase) and dehydrated back to periclase when the calcination temperature is raised to 600 degrees C. At this point, the transformation occurs simultaneously with changes in the textural and basic properties of the solid. Pt/MgO catalysts prepared by impregnation of magnesium oxide exhibit variable metallic and surface properties depending on whether Pt(NH3)(4)(NO3)(2) or H2PtCl6 is used as the impregnating salt.