Polymers have been identified as replacement materials for metallic liners in composite overwrapped pressure vessels (COPVs) for future space launchers. PEEK, Nylon, and PVDF plastics formed from base powder grades have been permeability tested to determine their susceptibility to the diffusion of helium through flatwise panel cross sections. Permeability, diffusion, and solubility coefficients have been obtained for each material with PVDF and PA11 grades showing the lowest permeability coefficients and hence the best barrier properties to permeation. Crystallinity percentages and internal air void contents in the polymer samples have also been used to assess the differences in permeability between materials with an analysis of void dispersion effects given through X-ray CT scanning techniques. The measured permeability coefficients have been used to assess the ability of all materials tested to act as a functional polymer liner in a standard COPV with final leak rates predicted based on liner thicknesses and weights. (c) 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43675.