Theoretical models of gas diffusion and permeation in microporous molecular-sieve membranes are presented The effect of the adsorbed diffusant on the total transmembrane flow is insignificant for permanent gases. For highly adsorbable gases the effect of the adsorbed molecules on the total transmembrane flux may be high at low temperatures. The activation energy of diffusion increases when the kinetic diameter of the diffusant increases. The activation energy of gas diffusion compares well with the values calculated based on the Lennard-Jones potential. Maximum possible permeability coefficients calculated for He in the molecular-sieve membranes do not exceed similar to 30,000 Barrer at room temperature. The experimentally observed value for Ne permeability is similar to 1,000 Barrer (T=30 degrees C) because of the high tortuosity (tau approximate to 25) and low porosity (theta=0.22) of the membrane porous structure.