Integral, isothermal heats of adsorption of CO on Pt dispersed on SiO 2, η-Al 2O 3, SiO 2-Al 2O 3, and TiO 2 have been measured using a uniquely modified differential scanning calorimeter integrated into a gas handling system. The Pt catalysts which had “normal” adsorption behavior exhibited a range of ΔH (ad) values at 300 K between 21 and 32 kcal/mole, with the more poorly dispersed Pt SiO 2 catalysts having the highest values and the very highly dispersed Pt TiO 2 , following a low-temperature reduction at 473 K, having the lowest values. A consistent trend occurred under “normal” adsorption conditions, independent of support, that indicated the presence of a crystallite size effect, with weaker COPt bonding occurring on small crystallites. Only the Pt TiO 2 samples after a high-temperature reduction, which showed the decreased chemisorption capacity associated with the strong metal-support interaction state, exhibited a pronounced support effect which reduced heats of adsorption at 300 K after repeated high-temperature reduction cycling to values as low as 10 kcal/mole. All ΔH (ad) values were lower at 215 K, and the range between 15 and 20 kcal/mole on the “normal” catalysts indicated the filling of lower-energy states. The logarithm of the CH 4 turnover frequency correlated almost linearly with the CO heat of adsorption over a 1000-fold variation in activity; however, this enhancement in specific activity cannot be easily explained by considering only a decrease in CO ΔH (ad), values.