In order to probe the thermal stability and reactivity of ethyl intermediates on Pt-Sn alloy catalysts, we have synthesized these species by reaction of H atoms with adsorbed ethylene on a ( ffiffi ffi (111) surface alloy. Adsorbed ethyl groups are stable until 376 K where they react to evolve ethane, ethylene, and H 2 . The activation energy for ethyl dehydrogenation is E dehyd à ¼ 97 kJ/mol, which is twice that reported on Pt(111). In addition, we place a lower limit of E hydr * > 70 kJ/mol on the barrier to ethyl hydrogenation on this alloy.