Microcalorimetric measurements of CO adsorption at 403 K on reduced Pt powder gave an initial heat of 140 kJ/mol and a saturation CO coverage of 30 mu mol/g Pt. The addition of metallic Rb and Cs to Pt increased the initial heats of CO adsorption by 20 and 40 kJ/mol and extended the CO adsorption capacity to 110 and 60 mu mol/g Pt, respectively. The increase in CO adsorption capacity is caused by formation of an alkali-CO complex at 403 K, with an interaction heat near 120 kJ/mol. Microcalorimetric measurements of CO adsorption at 403 K on Pt/SiO2 (0.85 wt % Pt) and Pt/Sn/SiO2 (2.61 wt % Pt) catalysts showed initial heats of 140 and 130 kJ/mol and saturation CO coverages of 25 and 65 mu mol/g, respectively. The addition of K, Ph, and Cs salts (1:5 atomic Pt/alkali) to Pt/Sn/SiO2 followed by treatment in H-2 at 773 K did not alter the properties of these materials for CO adsorption at 403 K or for reactions with isobutane at 673 K. In contrast, the addition of Na and Cs salts (1:3 atomic Pt/alkali) to Pt/Sn/SiO2 significantly decreased the CO saturation coverage, did not affect the initial heat of CO adsorption, and increased the selectivity for isobutane dehydrogenation. These results indicate that while the alkali species are not present in the metallic state on Pt/Sn/SiO2 catalysts, these alkali species are probably associated with Sn on Pt/Sn particles, thereby inhibiting isomerization, hydrogenolysis, and coking reactions by decreasing the size of surface platinum ensembles.