A Monte Carlo simulation of the Knudsen diffusion of gases in a porous solid is described and results of such simulations presented. The solid was represented in the computer as an assemblage of spheres. Equations are presented which enable the prediction of a Knudsen diffusivity from the mean pore size (or, preferably, the mean size and standard deviation) without resort to a tortuosity factor. © 1980 American Institute of Physics.