When a species diffusing in a porous solid is adsorbed to some extent, the ratio of adsorbed molecules to gas molecules in the pores may be very high if there isa large surface area associated with the pores. In addition to transport of molecules through the pores by gaseous diffusion, transport by surface diffusion may then become important. Good examples have been reported in the literature. However, there are many cases in which the mode of transport of an adsorbing species is limited to gaseous diffusion. Although the overall process is simpler when gaseous diffusion alone must be considered, the experimental diffusion coefficient determined directly by classical nonequilibrium methods is still not a true diffusion coefficient. Because of the complication of the "adsorption capacity factor", the experimental value is lower than the true value, and may be as much as orders of magnitude lower. For mesoporous solids this has been recognized by various investigators for some time. For certain microporous solids, namely zeolites with pore diameters of the order of a nanometer and frequently approaching the dimensions of very small molecules, there have been a number of reported examples where experimental values of diffusion coefficients obtained under nonequilibrium conditions are much lower than values obtained under equilibrium conditions by NMR. We suggest that the discrepancies in such examples can be understood if one attributes much of the total transport of molecules to a gaslike diffusion process in the pores of the zeolite. An adsorption capacity factor is then operative. For cases where experimental diffusion coefficients obtained under equilibrium and nonequilibrium conditions exhibit good agreement, which frequently involve zeolites with the smallest dimensions, we suggest that gas-phase diffusion no longer makes a contribution.