The continuous catalytic ammoxidation of propylene over the koechlinite phase of Bi2O3 · MoO3 has been studied in a fixed bed reactor under isothermal conditions (400 °C). The propylene concentration at the reactor inlet was varied over the range 1.04 - 2.15 mmol/liter and the ammonia concentration over the range 0.78 - 1.71 mmol/liter. The oxygen consumption rate was found to be independent of the propylene and ammonia concentrations. A quantitative kinetic model has been derived in which a surface reaction more or less inhibited by a product and oxygen diffusion through the lattice of the catalyst are combined. In this model a distinction is made between selective X-sites and nonselective Y-sites on the catalyst surface. The model provides a good description of the experimental results and also fits in with previous observations.