Porous solids have been simulated in the computer as assemblages of spheres. When such assemblages contain spheres distributed in size and randomly arranged in space, the structure of the resulting solid resembles that of a real porous solid. Monte Carlo calculations of gas molecule trajectories through the assemblages were carried out for both the Knudsen and ordinary diffusion régimes. Tortuosities calculated from the simulated diffusion “data” fell in the range obtained experimentally by other investigators. Correlations were obtained that enable the prediction of diffusion rates from measurement of the porosity, mean pore size, and standard deviation of the pore size.