Adsorption and hydrogenation of ethylene, 1-hexene, and benzene, and carbon monoxide adsorption on alumina-supported Pt and Pt Sn catalysts were studied. Gravimetric experiments and infrared (IR) absorption spectroscopy combined with chemisorption measurements were used for this purpose. Additionally fresh and coked catalysts were characterized by the IR spectra of CO and chemisorption data. Added tin causes a slight increase in benzene adsorption and a distinct decrease in ethylene and 1-hexene uptakes, whereas the hydrogenation activity is inhibited. Adsorption and catalytic data with benzene can be explained by a model of flat adsorption on Pt Al 2 O 3 and tilted adsorption on Pt-Sn Al 2 O 3 and are consistent with electronic modification of platinum by tin. Electronic interaction between platinum and tin is also indicated by the IR data. The most active sites for hydrocarbon decomposition on the platinum surface are the same as those on which carbon monoxide is multiply bonded. Deposited coke and tin block the same active sites on the platinum surface.