Rate studies were carried out for the para-ortho hydrogen conversion at 4O°C and I atmosphere with NiO on kieselguhr and CuO•Cr₂O₃ catalysts, separately. Data were obtained for different pellet sizes of catalysts, from which the effective diffusivities were determined. The catalyst pellets were tested for diffusion experiments and it was found that the effective diffusivity values under the nonreactive diffusion condition were considerably greater than the ones determined from the hydrogen conversion reaction. The effective diffusivities in the reacting system were evaluated from a parallel pore model and compared with the experimentally determined data.