The problem of calculating the diffusivities of gases in molecular sieves from experimental sorption curves is discussed. Commercial molecular sieve pellets contain zeolite crystallites of various sizes and it is shown that both the shape and the size distribution of the crystallites have significant effects on the sorption curves. For this type of system it is not possible to calculate reliable values of diffusivities using a mean equivalent spherical radius and in order to interpret the sorption curves cor-rectIy detailed knowledge of the crystallite shape and size distribution is required. Systems containing spherical and cubic particles are considered in detail and generalized sorption curves showing the effect of particle size distribution are presented. Experimental crystallite size distribution measurements are used to interpret typical sorption data for Linde 5A molecular sieve. The solution of the diffusion equation, modified to take account of the particle size distribution, fits the experimental data well suggesting that the basic assumption of Fickian diffusion is justified. It is suggested that some of the apparent anomalies in reported diffusivity data for molecular sieves may in fact be due to the neglect of the effects of crystallite shape and size distribution.