Sorption kinetics experiments were performed at 37 C with microcrystalline cellulose held between dryness and 51% water activity in the absence of other gases. Effective diffusion coefficients (<10−4 cm2/s) were greater than any previously reported for similar experiments. A model based on heat and mass transfer properties of the sample is proposed. The vapor space permeability calculated by application of this model is related to the structure of the microcrystalline cellulose as determined from water desorption isotherm analysis, mercury intrusion porosimetry, and steady state permeability techniques. Microcrystalline cellulose, like some freeze-dried foods, contains micropores and macropores. During sorption, the macropores are important in determining the balance between the internal and environmental control during the first half of sorption. The micropores, which account for less than 1% of the void volume of the porous matrix but approximately 40% of the surface area, control the mass transfer properties of the sample during the latter stages of sorption. Because of the great difference between their mass transfer properties, the macropores approach local equilibrium faster than the micropores; this phenomenon suggests that changes in the effective diffusion coefficient and permeability as sorption proceeds are related to the structure of the sample rather than to the moisture content itself.