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 (root3 x root3)R30degrees-Sn/Pt(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* = 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.