Ethylene adsorption on a family of Al2O3-supported Pt, Pt-Sn, and Pt-In catalysts was investigated at 300 K, The presence of Sn and In caused a decrease in ethylene uptake, similar to that observed in H-2 chemisorption measurements, Integral, isothermal heats of adsorption were measured and the heat of adsorption (Q(ad)) values for C2H4 on small Pt crystallites were typically around 21 kcal/mol and not markedly altered by the presence of Sn or In, In contrast, higher Q(ad) values of 30-31 of kcal/mol were obtained for C2H4 adsorption on large Pt crystallites with no Sn or In. These results are best explained by recognizing that on large particles decomposition of ethylene to form species with a C-E single bond, such as ethylidyne(of ethylidene), is favored, These reactions are exothermic and provide the higher Q(ad) values, This decomposition is suppressed on the smaller crystallites thus giving lower values consistent with those obtained from TPD of C2H4 from Pt single crystals which monitors only reversible desorption. This decomposition can also be suppressed by the addition of Sn pr In; but it has little effect with highly dispersed Pt, The heat of irreversible adsorption of C2H4 on Al2O3, In/Al2O3, and Sn/Al2O3 without Pt was 4.9, 6.6, and 9.5 kcal/mol, respectively.