Transparent membranes of chitin/poly(γ-methyl _L-glutamate) (PMLG) blends were obtained by casting their mixtures in trichloroacetic acid (35%)/1, 2-dichloroethane (65%) mixed solvent. Sorption and diffusion behaviors of water and methanol vapors into the membranes were discussed. Specific volume and adsorbed amount of water in the monolayer on the blends with various polymer compositions showed almost linear dependences on the volume fraction of chitin. This fact suggested a good compatibility of these polymers devoid of strong interactions between them. The interaction parameters between chitin and PMLG obtained from sorption experiments of methanol were nearly zero, which supported the suggested good compatibility. Clustering function showed that chitin has no clustering water over the whole range of humidity, while on PMLG and the blends water molecules begin to cluster at about 60% relative humidity. An empirical equation was obtained for the diffusion rate as the functions of the composition of the blends and the sorbed amount of solvent vapor. The equation revealed that the free volume of the blends is reduced by the mixing and that the diffusion rate is increased by the plasticizing effect of methanol on chitin.