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 increasing molecular mass of the chitosan. The state of water in chitosan films was analyzed in terms of various sorption and dissolution models: multimolecular adsorption (the BET theory), the Zimm-Lundberg cluster model, the Flory-Higgins lattice model, and the De Boer-Zwicker model. It was demonstrated that, over a wide range of vapor activities, water sorbs 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.