In an effort to better understand the excellent synergism of certain physical mixtures of supported catalysts for the l-butene double bond shift described previously, the butadiene hydroisomerization selectivity/activity of the same mixtures was studied. Limited activity/selectivity synergism for the hydroisomerization of butadiene was measured for physical mixtures of FeCe/Grafoil (alloy) and Pt/Grafoil or Pd/Grafoil (noble metal). A high degree of synergism was noted for mixtures in which the ratio of alloy to noble metal was less than about 20:1. Additions of alloy to mixtures which increased the ratio to values higher than 20 did not change the activity or selectivity. For Pd/Grafoil containing mixtures both activity and selectivity synergism could be explained completely on the basis of a hydrogen spillover model. For example, the limit in activity synergism was ascribed to the depletion of hydrogen atoms away from palladium centers (radial gradient around each noble metal particle) due to the use of these atoms in hydrogenation of butadiene on FeCe particles. In contrast, to explain the nonequilibrium ratio of a-butenes in platinum-containing mixtures, it is necessary to hypothesize two processes : both hydrogen spillover and bifunctional catalysis.