Selective introduction of bulky aryl substituents on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) leads to a 10-fold increase in PPO gas permeability. Aryl substitutions are obtained by reacting aryl-brominated PPO with an alkyne R-C=CH, using a Pd catalyst, where R is either Me(3)Si or Me(3)C. The gas permeability increase is a consequence of the packing density decrease and is quantitatively related to the composition of the derivatives, ranging from 15 to 85% repeat unit substitution. The Me(3)Si-C=C substituent is slightly more effective in increasing the permeability than the Me(3)C-C=C substituent; Me(3)C-C=C-substituted PPO raises the glass-transition temperature (T-g) towards higher temperatures while Si-containing groups leave T-g unchanged. Starting from this finding, a comparison has been made among the effects of several permeability-increasing functional groups on the PPO structure : PPO gas permeability can be enhanced by increases in both the intrasegmental stiffness and intersegmental distance. Depending on which of these is prevailing, the T-g of the product will accordingly increase or decrease.