A new approach for water vapour transmission rate (WVTR) measurement is demonstrated that uses an external ion beam. 'Sandwich' specimens were prepared comprising two films of the barrier material (PET) surrounding a thin film of hydrophilic poly(ethylene-oxide), "PEO". Deuterated water (D2O) permeating PET barrier films was trapped by the PEO, and quantified using He-3(2+) nuclear reaction analysis and proton detection. WVTR was determined from the rate of D2O accumulation in the PEO layer. This approach may be applied both to poor barrier films with WVTRs of > 200 g m(-2) day(-1), and may extend to films whose barrier properties exceed levels that can be characterised with conventional techniques. Variation in the proton energy distribution provides a means of measuring D2O migration within test samples, and could be used to detect the approach to failure in complex barrier films. In these experiments, D2O migration toward the irradiated face of the sample was attributed to ion beam-induced reactive sites or increased hydrophilicity of the barrier film, and was verified by separate measurements of the D2O content of PET films. Strategies for improving the sensitivity and accuracy of this method for WVTR characterisation towards a lower detection limit 1 mu g m(-2) day(-1) are discussed. (c) 2006 Elsevier B.V All rights reserved.