A modified Wicke-Kallenbach diffusion cell has been employed to measure the effective diffusivities of gases present in steam reforming in porous catalyst pellets within the temperature range of 293-873 K and at pressures from 1 to 10 bars. The temperature and pressure dependencies of the effective diffusivities of these gases have been evaluated and the tortuosities of the pellets estimated by the parallel path pore model. The experimental results demonstrated that diffusion in the present system occurs mainly in the bulk region at pressures up to 10 bars for the gases measured and, contrary to the usual predictions, the inverse proportionality of the effective diffusivity to the first power of the pressure is not always obeyed.