The isochoric (constant-volume, variable pressure) method for measuring permeation through membranes has been used extensively for several decades. In principle. this technique should be well suited for measuring the permeability of any gas separation membrane. In practice, accurate determination of the permeability becomes difficult as the membrane permeability decreases, due to outgassing and finite leakage of atmospheric gases into the permeate reservoir. To address these issues, an isochoric permeation system incorporating a novel membrane cell has been designed to minimize leakage. New masking techniques have also been developed for brittle materials and small film samples. (C) 2004 Elsevier B.V. All rights reserved.