The steady-state diffusion and flow of ternary mixtures of non-adsorbed gases through porous substrates cut from commercial pellets of nickel-based catalysts have been studied in a Wicke-Kallenbach cell. The rates of transport were obtained from the rates of flow of pure gases and composition measurements were obtained by gas chromatography. In several experiments a predetermined pressure difference across the cell was applied so that the measured rates of transport reflected the effect of transition-range diffusion and superimposed viscous flow, The effective binary diffusivities, Knudsen diffusivities, and the Darcy constant for viscous flow were estimated from the experimental measurements. A knowledge of these parameters is a prerequisite for modelling of the reactive catalyst.