The occurrence in mesoporous supports of a confinement effect related to the adsorption strength of a reaction intermediate on a metallic phase dispersed in the mesopores is examined, as well as its effect on the activity and selectivity. A model is proposed in order to determine the catalyst requirements (size of the pores and of the metal particles), which would allow to maximise the confinement. Alkyltrimethylammonium surfactants with increasing alkyl chain length were used to prepare MCM-41 supports, with controlled pore size between 22 and 39 angstrom and Pt degrees particles of controlled size (from 11 to 18 angstrom) 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-addition of four hydrogen atoms on o-xylene) on the metallic surface on the selectivity toward trans-dimethylcyclohexane (trans-DMCH) and (ii) the kinetic diameter of DMCHE (6.4 angstrom) is close to the distance between the metallic particle surface and the silica wall of the series of Pt degrees/MCM-41 catalysts. Slight decrease in the turn-over frequency (TOF) and increase in the selectivity toward trans-DMCH are observed for the smaller pore sizes, and the assignment of this effect to a confinement of the reaction intermediate is discussed.