Bimetallic catalysts involving Sn and Pt have important applications in hydrocarbon conversion catalysis, but little fundamental information is known about the chemistry and catalysis that occurs at distinct Pt-Sn alloy phases, how this chemistry varies with composition and structure, whether various phases are optimal or detrimental to catalyst performance, and whether novel nanostructures have special chemical properties. The need for improved selectivity of catalysts, and improvements in catalyst performance overall, drive our studies. We have carried out a wide range of investigations probing chemisorption and reaction kinetics on well-defined, bimetallic Pt-Sn surfaces in order to improve basic understanding of the chemistry of hydrocarbon molecules and reactive intermediates on such surfaces. I will summarize our results, focusing on selective dehydrogenation and hydrogenation reactions, and briefly discuss more generally the role of a second, added metal component in chemistry and catalysis on alloy surfaces.