Ordered bimetallic Sn/Pt(111) surface alloys have been prepared in an ultrahigh vacuum system and used in model moderate pressure (1-200 Torr) batch catalytic reactions. Hydrogenolysis of n-butane (H-2/n-C-4 = 100) was conducted on these surfaces to characterize the effects of ordered bimetallic ensembles relative to those available at the Pt(111) surface over the temperature range 525-625 K. For these conditions, specific n-butane hydrogenolysis reaction rates were determined for the p(2 x 2) Sn/Pt(111) surface alloy, the (root 3 x root 3)R30 degrees Sn/Pt(111) surface alloy, and Pt(111) as a function of temperature. The order of relative activities is p(2 x 2) Sn/Pt(111) surface alloy > Pt(111) > (root 3 x root 3)R30 degrees Sn/Pt(111) surface alloy with specific activation energies of 29 +/- 2 kcal mol(-1) for all three surfaces below 575 K. A leveling in activity is seen for all three surfaces at temperatures above 575 K. The hydrogenolysis activity of the p(2 x 2) Sn/Pt(111) surface alloy is approximately 5-6 times greater than that seen over Pt(111) and the (root 3 x root 3)R30 degrees Sn/Pt(111) surface alloy is approximately an order of magnitude less active. Product distributions and catalytic deactivation were found to be substantially different for these three surfaces. These results are discussed in terms of the ensemble requirement differences inherent to the hydrogenolysis pathways available at each of the base metals under these conditions of temperature, pressure, and hydrogen/hydrocarbon ratio.