The gas-phase hydrogenation of acetonitrile was studied with silica-supported platinum catalysts of which the degrees of metal dispersion were widely changed by reduction conditions. The activities were found to decrease gradually during the course of reaction for all the catalysts examined. The initial rate of reaction increased with an increase in the degree of platinum dispersion, D. Triethylamine was the only main product irrespective of D and period of reaction time. The initial turnover frequency. TOF0, was shown to be smaller for larger D values. This dependence of TOF0 on D was explained by the electronic state of the surface of the platinum particles and the state of acetonitrile molecules adsorbed on them on the basis of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy measurements. The surface layer of smaller particles is more favorable for the adsorption of acetonitrile. The acetonitrile is adsorbed by platinum with the electron lone pair of nitrogen in the antibonding orbital, but electron back-donation does not takes place. As a result, the C=N bonds of acetonitrile adsorbed on smaller particles are stronger and more difficult to hydrogenate.