Powders of silver catalyst of low calcium content and an 8.5% calcium in silver alloy were pelletized at different compacting stresses. The diffusibility of each pellet was measured, using hydrogen, and correlated with internal void volume, The surface area of the powders and of some representative pellets was obtained by nitrogen adsorption. All these parameters were used in an attempt to interpret the changes taking place with decreasing porosity. With the help of two plausible assumptions it was concluded that the tortuosity remained constant over the range examined. The figures obtained have limited usefulness for the prediction of approximate pellet diffusibilities from a knowledge of the compacting stresses applied to similar metal powders.