Gas transport properties of three different PEO-based polymer electrolytes containing PEO sulfonic acid dianions, cations and 90 or 35 or 0 wt% of water are studied by atomistic molecular modelling. The gas molecules studied are H-2, O-2, CO2 and CH4. The pair correlation functions are calculated to see the distribution of the gas molecules in the systems. The diffusion coefficients of the gas molecules are calculated and found to be dependent of the amount and distribution of the water in the system and of the size of the penetrant. The motion of the gas molecules is explored in each system. The amount and distribution of water has a strong effect on the motion of the gas molecules. The ionic copenetrants seem not to have any direct influence on the diffusion of the gas molecules in the polyelectrolyte materials. However, ions have an effect on the distribution of the water in the polyelectrolyte materials. (C) 2004 Elsevier Ltd. All rights reserved.