Butadiene is an impurity in C4 alkene streams produced by steam cracking, and it is desirable to selectively hydrogenate this butadiene to butene. Unfortunately, much butane is usually produced over platinum catalysts used for this purpose. Supported Pt-Sn bimetallic catalysts have been reported to be effective in selectively hydrogenating diolefin impurities. The presence of Sn in bimetallic Pt catalysts has also been reported to prevent alkene hydrogenation, which is important for selectively removing diene impurities. We discuss our investigations of butadiene hydrogenation on well-defined model catalysts by coadsorption of hydrogen and butadiene under UHV conditions on Pt(111) and two ordered Sn/Pt(111) surface alloys. This chemistry was characterized by using temperature programmed desorption (TPD). In particular, we address the influence of alloyed Sn on the reaction barrier to butadiene hydrogenation and the effect of surface Sn concentration on hydrogenation activity and selectivity.