Current/potential responses of small organic molecules show as a characteristic feature an increase in anodic current when changing the potential in the cathodic direction. In situ infrared data on the adsorption of methanal, formic acid, and water at Pt(111) are used to develop a new model explaining these well-known catalytic effects. Since the strength of water adsorption increases with increasing potentials above 0.4 V, the adsorption of organic molecules has to be considered as a water displacement reaction. The reaction rate is thus the result elf the concurrence of two processes that are oppositely affected by the potential : the rate of oxidation (charge transfer) and the rate of adsorption. In situ TX data and cyclic voltammograms for the oxidation of methanal and formic acid on a Pt(111) surface are presented and discussed. For comparison, data on CO oxidation are presented.