The adsorption and desorption of carbon monoxide and water as well as interaction between them on polycrystalline platinum has been studied using the technique of thermal desorption spectroscopy（TDS） under ultrahigh vacuum (UHV) conditions. A small amount of water adsorbs on the surface of the platinum at 310K and desorbs readily and steadily in the temperature range of 350-450 K. When exposure of water by more than 100 L is followed by saturation exposure of carbon monoxide, the height of carbon monoxide desorption peak appeared at 790 K becomes higher. It suggests that preadsorbed water dissociates a small part of carbon monoxide to atomic carbon and oxygen on the surface of the samole. And, interaction of water and carbon monoxide (or atomic carbon) results in the water-gas shift reaction.