The characterization of manganese-lanthanum oxides modified with silver has been performed in order to identify factors responsible for the variation of their activity in the oxidation of methane. A significant increase in the activity per unit surface area in silver-containing catalysts occur-red above 800 K, where a new source of surface oxygen appeared. It is probably oxygen released from filled oxygen vacancies, more weakly bound in the oxides structure in comparison with lattice oxide ions, more mobile, and therefore easily accessible to methane oxidation. Such oxygen is probably neighboring with silver ions. The remaining part of the catalyst may constitute a reservoir of oxygen ions with which the vacancies are filled and which is supplemented with the gaseous oxygen. A consequence of filling up oxygen vacancies is the appearance of a larger number of manganese ions in the unstable oxidation state Mn4+. The rate of methane oxidation is a function of the Mn4+/Mn3+ Surface ratio which is a parameter characterizing the intrinsic properties of the manganese-lanthanum oxides, influencing their activity. (C) 2004 Elsevier Inc. All rights reserved.