We have investigated the surface reactivities of Ni/Pt(111) bimetallic model catalysts using ethylene and cyclohexene as probing molecules. The bimetallic surfaces were generated by evaporating Ni onto a Pt(111) single-crystal surface held at 600 K. The surface chemistry was investigated using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED). The reactivities of the bimetallic surfaces were compared with those of the clean Pt(111) surface and a thick Ni(111) film on the Pt(111) substrate. Formation of the bimetallic surface led to a significantly reduced reactivity towards the decomposition of ethylene when compared to either Pt(111) or Ni(111)/Pt(111) surfaces. Furthermore, although the surface reactivity towards cyclohexene was retained for the bimetallic surface, the decomposition mechanism was distinctly altered from that of either Pt(111) or Ni(111)/Pt(111) surfaces.