Parameters of interaction between chitosan and carboxymethylcellulose with varying molecular masses and degrees of deacetylation and carboxylation were measured by viscometry. For one pair of polymers, the effect of medium acidity on the formation of an interpolymer complex by mixing of polymers in solutions was examined in detail. The existence of the complex was confirmed by X-ray diffraction. Sorption and diffusion properties of films differing in the content of chitosan in the complex were studied by the sorption analysis in a wide range of water vapor pressures. Sorption isotherms and kinetic curves of film swelling in water were obtained. It was shown that the films prepared from complexes formed in 0.05 N hydrochloric acid sorb similar to 2-2.5 times less the amount of water than the films of the same composition prepared in acetate buffer. On the contrary, coefficients of diffusion of water molecules in films with different chitosan : carboxymethylcellulose ratios as calculated from the kinetic profiles of sorption were found to be higher for samples prepared from hydrochloric acid solutions. The experimental relationships were explained considering the effects of the acidity of the medium on the degrees of ionization of chitosan and carboxymethylcellulose and on the structure of the complex formed upon mixing of solutions and the influence of hydration of ions in macromolecular chains not involved in interpolymer salt bonds on diffusion processes.