Phosphonium poly(ionic liquid)s (PILs) have been studied as alternatives to more common ammonium and imidazolium PILs for potential transport and separation applications. This work characterizes the CO2, H-2, N-2, O-2, CH4, and C2H4 single-gas permeability, diffusivity, solubility, and selectivity of freestanding films of poly([(tri-n-alkyl)vinylbenzylphosphonium][bis(trifluoromethylsulfonyl)imidel) PILs (i.e., poly([P-nnnVB][Tf2N) where n=4, 6, 8). The gas permeability was found to increase approximately linearly with increasing alkyl chain length on the phosphonium group. To our knowledge, the CO2 permeability of 186 barrers observed for poly([P888VB][Tf2N]) is the highest reported for neat PIL materials. In contrast, gas selectivity was observed to decrease with an increase in phosphonium alkyl chain length from n = 4 to n = 6, then remain approximately constant between n = 6 and n=8. Additionally, the ionic conductivity of these materials was observed to increase from ca. 10(-8) to ca. 10(-5) S cm(-1) as the measurement temperature was increased from 25 to 105 degrees C. At 25 degrees C, the PIL with the shortest cation alkyl chain (n=4) was observed to have the lowest ionic conductivity. However at ca. 90 degrees C, the expected trend of increasing ionic conductivity in the order n=4 > n =6 > n=8 was observed. (C) 2015 Elsevier B.V. All rights reserved.