A steady-state film model, accounting for the species diffusion inside the catalytic layer, was developed. This model demonstrates that diffusional limitations within the washcoat limit the propene conversion. The effectiveness factor was found lower than 1 at any location along a channel of the catalytic converter, for inlet temperatures higher than 500 K. These diffusional limitations within the washcoat had to be accounted for to extract the kinetic rate of propene oxidation in agreement with the experimental measurements of the extent of propene conversion at different temperatures. The effectiveness factor is not affected by a transient state since the characteristic time scale for internal diffusion is much less than the period of fluctuation of the gas composition. The effective diffusivities of propene and oxygen are key parameters for the efficiency of the propene conversion. The platinum content is less determining since the concentrations of propene and oxygen significantly decrease along the thickness of the washcoat. A study of the external mass transfer, affected by the hydraulic diameter, showed that a decrease in the channel diameter significantly increases the extent of propene conversion for temperatures higher than 500 K.