In order to explain the high stability of the Co- and Ni-MCM-41 catalysts, the hypotheses of anchoring of small metallic clusters to cations in and partial occlusion of the metallic clusters by amorphous silica are proposed. Carbon monoxide chemisorption on model catalysts and Co-MCM-41 has given direct proof of the metal anchoring effect on the stability of metal cluster dispersion. The partial occlusion of metallic clusters, which may lead to physical confinement (inhibited migration) or chemical confinement (inhibited reaction), was monitored by CO in situ FTIR and CO methanation along with a kinetic study, which confirmed that the metallic clusters on the pore surface result in the stabilization of the catalysts against deactivation. A fine tuning of these physicochemical phenomena in the transition metal incorporated MCM-41 will make possible the design of stable catalytic systems for reactions requiring metallic active sites under severe reaction conditions. (c) 2007 Elsevier B.V. All rights reserved.