The adsorption of ethyl isocyanate from the gas phase onto silica surfaces has been investigated by infra-red spectroscopy. Products of the adsorption process include a surface urethane, 1,3-diethylurea, a biuret, dissociatively adsorbed isocyanate and ethoxy groups, and compounds formed by the polymerization of isocyanic acid and hydrogen cyanide. The relative proportions of the reaction products are sensitive to the temperature of adsorption and the temperature at which the silica surface was evacuated before exposure to the adsorbate. The formation of diethylurea only occurs when molecular water or hydrogen-bonding hydroxyl groups are present on the oxide surface. The rate of formation of diethylurea from liquid mixtures of ethyl isocyanate and water is considerably enhanced by the addition of powdered silica. The hydrolysis reaction, promoted by silica, provides a method for the rapid preparation of 1,3-diethylurea.