Sorption and diffusion of water vapors in films of chitosan acetates of varying molecular mass were studied using a McBain microbalance. Based on sorption isotherms, the Gibbs energies of mixing of polymers with water were calculated. It was found that these parameters increase with the increasing molecular mass of chitosan. The state of water in chitosan films was analyzed in terms of various sorption and dissolution models: multimolecular adsorption (the BET theory), Zimm-Lundberg cluster model, Flory-Huggins lattice model, and De Boer-Zwicker model. It was demonstrated that, over a wide range of vapor activities, water is sorbed on the active groups of the polymer and does not yield clusters. The coefficient of water diffusion in chitosan films grows, as the concentration of water in the system increases.