The di- and trimerization reactions of acetylene were studied over Pt(111) and (root 3x root 3)R30 degrees-Sn/Pt(111) model catalysts at moderate pressures (20-100 Torr, hydrogen-hydrocarbon ratio of 10). The catalyst surfaces were prepared and characterized in a UHV surface analysis system and moderate pressure catalytic reactions were 1/3 conducted with an attached batch reactor. The overall catalytic activity of the (root 3x root 3)R30 degrees-Sn/Pt(111) surface alloy was similar to-4-5 times higher than that of Pt(lll). Both surfaces produced only C-4 and C-6 hydrocarbons as di- and trimerization products with C-4 production rates being about an order of magnitude higher than that for C-6 hydrocarbons. Besides the di- and trimerization reaction, hydrogenation of acetylene into ethylene was also observed. Among the C-4 products, butadiene, l-butene, and n-butane were the major components. Both linear and cyclic C-6 hydrocarbons were produced. Among the linear C-6 products paraffinic (n-hexane), olefinic (l-hexene), and diolefinic (hexadiene) hydrocarbons were observed. The main components of the cyclic C-6 products were cyclohexane, cyclohexene, 1,3-cyclohexadiene, and benzene. For both product groups the degree of unsaturation of the hydrocarbon molecules depended upon the experimental conditions applied (P-H2/P-C2H2; T). The formation of carbonaceous surface residues was seen under all experimental conditions. The di- and trimerization of acetylene was not eliminated by the presence of surface carbonaceous deposits and even at a high level of carbon buildup the catalysts exhibited significant activities. The very good correlation found between the formation rates of butadiene and cyclic C-6 hydrocarbons suggests that the formation of ring C-6 products proceeds through a metallocyclopentadiene intermediate. This species can either be hydrogenated off from the catalyst surfaces to produce butadiene or be reacted with a third acetylene molecule to form ring C-6 hydrocarbons.