A water-specific permeameter was developed to study water diffusion in an unsaturated polyester; these polyesters are often used as high-barrier materials. Low water enrichment of a dry sweeping gas is measured via the dew point temperature, with two hygrometers, one of which is used because of its fast response and the other because of its accuracy. This high-performance device is just as suited to pervaporation as to permeation tests and allows the transient and stationary fluxes to be characterized. At first sight, the experimental data seem in good agreement with D = D(0)e(gamma C). However, a more thorough study has shown a time-dependence of D.