The dimerization reaction of ethylene was studied over Pt(111) and (square-root3 x square-root3)R30-degrees-Sn/Pt(111) model catalysts at moderate pressures (20-100 Torr). The catalyst surfaces were prepared and characterized in a UHV surface analysis system and moderate pressure catalytic reactions were conducted with an attached batch reactor. The overall catalytic activity of the (square-root3 x square-root3)R30-degrees-Sn/Pt(111) surface alloy for C4 products was slightly higher than that at Pt(111). In addition to the dimerization reaction, hydrogenolysis of ethylene to propane and methane was also observed, with the (square-root3 x square-root3)R30-degrees-Sn/Pt(111) surface alloy less active than Pt(111). Among the C4 products, butenes and n-butane were the major components. Carbon buildup was observed to be significant above 500 K with the (square-root3 x square-root3)R30-degrees-Sn/Pt(111) surface alloy much more resistant than Pt(111). The dimerization of ethylene was not eliminated by the presence of surface carbonaceous deposits and even at significant surface coverages of carbon the model catalysts exhibited significant activitis. The results are discussed in terms of the surface chemistry of ethylene and the previously reported catalytic reactions of acetylene trimerization and n-butane hydrogenolysis at these surfaces.