A silica-supported Ag catalyst has been shown to be an efficient heterogeneous catalyst for the oxidant-free dehydrogenation of ethanol into acetaldehyde. The reaction mechanism has been investigated by in situ FTIR spectroscopy. The kinetic isotope effects for proton and hydride abstraction have been studied by using CH3CD2OH and CH3CH2OD as labeled reactants. The results indicate that OH bond activation and the formation of a hydrogen-bonded complex take place on the silica support and that the Ag particles are necessary for the activation of the CH bond. The kinetic isotope effect (kH/kD) is 1.9 for CH3CD2OH and 1.8 for CH3CH2OD. The concerted mechanism of CH cleavage on the Ag sites and proton abstraction on the silica sites is proposed to account for the results of the spectroscopic and kinetic experiments.