The effective diffusion coefficient of methane in nitrogen is investigated in the washcoat of a honeycomb type monolith reactor. A flow cell operating at atmospheric pressure is used. The effective diffusivity of methane in the alumina based washcoat is found to lie in the range 0.6-4.2 x 10(-7) m(2)/s, whilst the value in the cordierite support is 9.2 (+/-3.8) x 10(-7) m(2)/s. The experimental values are compared to those obtained from the random pore model, and are found to be significantly smaller. So, the random pore model is not appropriate for this alumina washcoat. The parallel pore model could be used to model the results. The tortuosity factor required to model the results using the parallel pore model was found to be 8.5 and 8.1 for the cordierite and washcoat, respectively.