Mn(TMPyP) metalloporphyrin catalysts supported on aluminated MCM-41 and SBA-15 mesoporous silicas were investigated in the liquid phase oxidation of cyclooctane with molecular oxygen (as air) without the use of sacrificial co-reductant. Cyclooctanone and cyclooctanol were the only reaction products, the first one appearing in overwhelming majority. The activity of the catalysts increased with the increasing pore size of the support, which pointed to the existence of diffusion limitations within the pore inner space. An exceptionally high selectivity to cyclooctanone observed for the MCM-41-derived catalysts, as opposed to SBA-15-based ones and to homogeneous oxidation process, is tentatively interpreted in terms of steric effect due to the suppression of the hydroperoxide intermediate formation in relatively narrow channels of MCM-41. Catalysts show good stability in the reaction conditions. (c) 2006 Elsevier B.V. All rights reserved.