A series of manganese oxides loaded on an active magnesia support was prepared by the calcination at 400-800degreesC of magnesium carbonate impregnated with manganese nitrate solution. The extent of loading was varied between 16.55 wt.% and 61.34 wt.% Mn2O3. The samples obtained were characterized for the crystalline phase structure by X-ray powder diffractometry, the surface area by BET-analysis of nitrogen adsorption isotherms and the H2O2 decomposition activity by oxygen-gasometry of the reaction kinetics at 20-40degreesC. The results revealed that treatment of the Mn/Mg mixed oxide system with increasing amounts of manganese oxides followed by calcinations at 400-800degreesC brought about an increase in the catalytic activity of the resulting solids, whilst the opposite effect was observed in the surface area of these solids. This treatment resulted in an increase in the particle size of MgO and a decrease in both the activation energy of sintering of the investigated system and that necessary for H2O2 decomposition reaction. (C) 2004 Elsevier B.V. All rights reserved.