Platinum catalysts prepared on activated carbon supports of different origin are compared in the hydrogenation of cyclohexene and cyclooctene, which differ much in their reactivity. A decrease in the catalytic activity upon decreasing the dimensions of the micropores in the support and the size of the supported metal particles has been confirmed. The results give evidence against a trivial role of the diffusional factor in such an effect and indicate complete blocking of part of the metal surface in narrow pores.