The understanding of water-polymer interactions in polymeric composite materials is critical to the prediction of their behavior in applications where they are exposed to water or humid environment. Moisture diffusion in unsaturated polyester-woodflour composites exposed to environments kept at room temperature and different relative humidities was investigated. Equations obtained from microscopic mass balances for vapor diffusion in solids were used to determine the effective diffusion coefficients of the different woodflours, neat resin and composites. The effect of the size and shape of the composite specimens used in the experimental work on the final moisture content was also evaluated. Different models were used to predict the composite effective diffusion coefficients as a function of filler concentrations.