Microcalorimetric measurements of the differential heats of hydrogen and carbon monoxide adsorption versus adsorbate coverage were made at 403 K for platinum supported on silica, magnesia/alumina, L-zeolite, Y-zeolite, and ZSM-5. The differential heats at zero coverage for hydrogen and carbon monoxide adsorption were 90 and 140 kJ/mol, respectively. for platinum supported on silica and nonacidic zeolites. The differential heats were larger by approximately 20 kJ/mol for hydrogen and carbon monoxide adsorption on platinum particles supported on basic supports such as potassium/silica, magnesia/alumina, and zeolites containing basic cations (K+, Ba2+) exchanged in excess of the zeolite framework aluminum content. The microcalorimetric results suggest that the high paraffin aromatization activity and selectivity observed for L-zeolite-supported platinum catalysts do not appear to be caused solely by changes in the adsorptive properties of the cluster-sized platinum particles located within the zeolite.