The decomposition of ethylene and the self-hydrogenation of cyclohexene have been used as probe reactions to compare the activities of Ni/Pt(111) and Pt/Ni(111) surfaces. Ethylene decomposes on both pure Pt(111) and Ni(111) surfaces. Both the 1 monolayer (ML) Ni/Pt(111) and 1 ML Pt/Ni(111) surfaces exhibit a decrease in activity toward ethylene decomposition as compared to either Pt(111) or Ni(111). At higher metal coverages (similar to3 ML), however, the Pt/Ni(111) surface remains less active, but the Ni/Pt(111) surface becomes Ni-like toward ethylene decomposition. The comparative study of cyclohexene reaction pathways further reveals the different activities between the similar to3 ML Pt/Ni(111) and similar to3 ML Ni/ Pt(111) surfaces.