H-2 and CO2 transport properties were investigated on semicrystalline poly(ethylene-2,6-dicarboxylate naphtalene) (PEN) films and biaxially stretched films and analyzed in terms of solubility and diffusion. The decrease of the permeability observed on the thermally crystallized samples has been described by Maxwell's law. No change of the sorption properties of the amorphous phase has been noticed as a function of the thermal treatment and the evolution of the diffusion coefficient has been related to a tortuosity effect. The low permeability measured on the biaxially stretched film is related to both a change of the free volume sizes distribution and a tortuosity effect. The interesting barrier properties of the biaxially stretched film are kept even after annealing the film at 250degreesC. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1849-1857, 2003.