A platinum-loaded H-mordenite (PtHM) catalyst has been modified by the chemical vapour deposition of silicon alkoxide in order to improve its shape-selectivity for the hydrocracking of paraffins. The paraffins reacted on the silica-coated PtHM according to their molecular size, and reactant shape-selectivity was achieved by choosing the extent of modification. Silica was only deposited on the external surface of the zeolite to form mono-, di- or tri-layers of SiO2, with the internal acid and metal sites unpoisoned, thus effectively narrowing the pore exit size only. Platinum metal on the external surface was not completely covered by the silicon oxide.