A kinetic study of the reduction of nitric oxide by ammonia in the presence of oxygen on a novel catalyst prepared as a physical mixture of V2O5-WO3/TiO2 catalyst and a natural silicate (sepiolite) was carried out. The steady-state differential reactor data were fitted to several equations using a discriminating method based on physical and statistic criteria. The best fit was obtained from a model involving adsorption of ammonia and oxygen which react with nitric oxide from the gas phase following an Eley-Rideal mechanism. The equation considered for this model reproduces well the experimental results obtained in a differential regime. Important limitations by internal diffusion were determined when the reaction took place in an integral regime.