Stable ordered structures of Sn on Pt(111) (maximum coverage of 0.33 monolayers) were used as model catalysts to test the effect of tin and potassium on the hydrogenation of ethylene at 300 K and dehydrogenation of cyclohexane at 573 It and at pressures of 15 Torr of hydrocarbon and 100 Torr of H-2. Co-adsorption of tin and potassium on Pt(111) resulted in the direct interaction between potassium, tin, and platinum as verified by temperature-programmed desorption of CO. Tin deposition yielded a maximum in the turnover rate as a function of Sn coverage for ethylene hydrogenation and cyclohexane dehydrogenation with maxima at about 0.2 monolayers of tin and a turnover rate 75% higher than on clean Pt(111), This enhancement was explained by a lower rate of deactivation as tin was added. In contrast, the addition of potassium to Pt and Pt/Sn produced only a monotonic decrease in cyclohexane dehydrogenation. In an industrial system, where a higher tin coverage is used, interaction of tin with potassium may form an effective site blocker which could lower deactivation rates.