The occurrence in meso-porous supports of a confinement effect related to the adsorption strength of a reaction intermediate on a metallic phase dispersed in the meso pores is examd., as well as its effect on the activity and selectivity. A model is proposed in order to det. the catalyst requirements (size of the pores and of the metal particles), which would allow to maximize the confinement. Alkyltrimethylammonium surfactants with increasing alkyl chain length were used to prep. MCM-41 supports, with controlled pore size between 22 and 39 .ANG. and Pt Deg particles of controlled size (from 11 to 18 .ANG.) were formed in the porosity of the MCM-41 supports. The reaction of hydrogenation of o-xylene was chosen as model reaction, because (i) previous studies have highlighted the influence of the adsorption strength of 1,2-dimethylcyclohexene (DMCHE, reaction intermediate formed by cis-addn. of four hydrogen atoms on o-xylene) on the metallic surface on the selectivity toward trans-dimethylcyclohexane (trans-DMCH) and (ii) the kinetic diam. of DMCHE (6.4 .ANG.) is close to the distance between the metallic particle surface and the silica wall of the series of Pt Deg/MCM-41 catalysts. Slight decrease in the turn-over frequency (TOF) and increase in the selectivity toward trans-DMCH are obsd. for the smaller pore sizes, and the assignment of this effect to a confinement of the reaction intermediate is discussed.