A detailed analysis of the sorption equilibrium and the diffusion of water vapor under different activities through an unsaturated polyester resin (UPR) has been undertaken by differential permeation and microgravimetry techniques. The BET-type III sorption isotherm obtained by microgravimetry was analyzed with the Zimm-Lundberg approach to determine the mean cluster size in the UPR film: the latter increases drastically with the water content in the film. The transient permeation Aux can be well fitted when a concentration-dependent diffusivity of exponential type is used. From the water content at sorption equilibrium and the steady-state permeability, a mean diffusion coefficient for the steady state can be determined; its decrease with increasing water activity is consistent with the increase in the mean cluster size.