The adsorbate formed after adsorption of cyclohexene on Pt(110) and polycrystalline Pt electrodes is displaced by competitive adsorption of UPD copper. Since it desorbs as benzene, cyclohexene is dehydrogenated during adsorption. After adsorption of ethene on polycrystalline Pt, only 10% of the adsorbate are displaced by Cu, leaving the surface as ethene. This corresponds to weakly adsorbed species, probably pi-bound ethene.