The CO poisoning effect on the catalytic ethylene hydrogenation reaction on the Pt(1 1 1) surface has been studied by surface vibrational sum frequency generation (SFG) technique. It was found that a monolayer of CO prevented the adsorption of ethylene on the Pt(1 1 1) surface in the presence of high pressure ethylene (up to 35 Torr). In contrast. CO can adsorb on the ethylene pretreated Pt(1 1 1) surface by increasing the CO pressure. However, the coadsorbed ethylidyne molecules weakened CO bonding, which was evidenced by the red-shift in the vibrational frequency. In the presence of high pressure CO (>0.1 Torr) the catalytic ethylene hydrogenation was poisoned due to the site blocking effect of CO adsorption. No catalytic reactivity was observed at temperatures lower than the CO desorption temperature (400 K). The measured apparent activation energy of the CO poisoned ethylene hydrogenation reaction was about 20.5 kcal/mole.