A reaction rate model based on complete wetting of the catalyst particles in a trickle-bed reactor is presented. It is considered that the pendular rings are formed at the contact points of the particles and the liquid flows down as thin films on the remaining surface between the pendular rings. Assuming that the flow in the films is laminar, the liquid in the pendular rings is well-mixed, and a fraction of the liquid flowing in the films bypasses the pendular rings, the reaction rate was found. The data obtained for oxidation of sulfur dioxide on activated carbon were used to test the model predictions of the dependence of reaction rate on liquid velocity. The reaction rates in nonprewetted beds were found to be lower than those in prewetted beds. The proposed model correlated the observed reaction rates satisfactorily, with the fractional thickness of the film that bypasses the pendular ring as an adjustable parameter. The model also predicted the reaction rate trends, including the minimum of the oxidation rate reported by Mata and Smith (Chem. Eng. J. 1981, 22, 229-235).