The catalytic activity of commercial CaO and MgO and their physical mixtures on the gasification reactions with steam was studied. Steam gasification of naphthalene as model reaction was selected and experiments in a laboratory scale reactor were performed. Carbon conversion for pure MgO and CaO was 54 and 62%, respectively. Every mixture showed a larger conversion than the weighted average of the pure compounds conversions; i.e., they exhibited catalytic synergy. The largest catalytic activity, reflected by the carbon conversion (79%), was obtained with a 10% CaO + 90% MgO mixture, which also showed the highest catalytic synergy (44.2%) among the assayed mixtures. Characterization of catalysts by XPS, XRD, SBET and DTG/DTA lead to conclude that formation of new phases, sintering, modification of dehydration or carbonation of the oxides in the mixture could not explain synergy, or basicity changes. Oxides forming the mixture have not been modified by the preparation or during the catalytic gasification. A catalytic cooperation between the two separated oxides is observed. Thus, synergy is attributed to MgO presence in the mixture, which inhibits formation of carbonaceous material as well as bidentate carbonate in the CaO surface, while promotes the formation of unidentate carbonate of lesser stability at the reaction temperature and in the presence of H2O(v). (C) 2004 Elsevier B.V. All fights reserved.