AbstractConstant pressure diffusion rates of hydrogen through Vycor porous glass were measured in nitrogen and neon systems. The experimental diffusivities are less than those predicted by the random pore model indicating that this model may not be satisfactory for a low porosity material such as Vycor. Interpreted in terms of an assembly of parallel pores the data suggest a tortuosity factor of approximately 2.4.Rate data for the ortho‐para hydrogen reaction were also measured at −196°C. and 1 atm. The catalyst 2% nickel/Vycor was found to have pore properties (area, mean pore size, pore volume) similar to that of plain Vycor. Predicted effectiveness factors using the experimental diffusion data were in good agreement with values determined experimentally. The latter were evaluated from rate measurements on both pellet and particles (58 μ mean diameter).