The reactions of hydrogen, ethylene, and cyclohexene have been investigated on Co/Pt(111) surfaces using temperature-programmed desorption (TPD). When Co is deposited on Pt(111) at 0.5-1 ML (monolayer) Co coverage, hydrogen desorption occurs in the temperature range of 150-250 K, which is significantly lower in temperature than from pure Pt(111) or a thick Co film on Pt(111). The ethylene decomposition reaction pathway is also significantly decreased on the 0.5-1 ML Co/Pt(111) surfaces. In addition, cyclohexene reaction pathways on 1 ML Co/Pt(111) are different from pure Pt(111) or thick Co films. The only reaction pathways of cyclohexene on Pt(111) or thick Co films are dehydrogenation to benzene and complete decomposition. However, on the similar to1 ML Co/Pt(111) surface, a new hydrogenation reaction pathway to cyclohexane is present at 221 K. These reactions on the Co/Pt(111) surfaces are further compared to our earlier studies of the Ni/Pt(1 11) surfaces. The novel activity of the I ML Co/Pt(111) surface is explained by the modification of the Pt d-band center by the subsurface Co atoms.