A model edible film made with methylcellulose was studied for its water vapor barrier properties. The steady-state water vapor transmission rate (WVTR) increased with both the water vapor pressure gradient and the initial water content before permeation. A decrease in water diffusion with increasing moisture content was due to a clustering phenomenon of water molecules within the film. Water concentration profiles within the film were estimated from the sorption isotherm, and differed from the theoretical linear profile based on Fick’s first law. The WVTR and the diffusivity depended strongly on the water concentration because of interactions between water molecules and the polymer matrix. Thus water vapor permeability (WVP) calculations used for synthetic polymeric packaging and based on Fick’s and Henry’s laws do not apply for methylcellulose edible films.