High molecular weight PPO was brominated to three levels of degree of bromination. Each brominated PPO was further sulfonated to three levels of degree of sulfonation. Membranes were prepared from these sulfonated brominated PPO (SPPOBr) and the transport properties of CO2, CH4, O-2, and N-2 gases in these membranes were characterized. The main effect of bromination of PPO is to increase the permeability of gases while maintaining the same perm selectivity. The main effect of simultaneous sulfonation and bromination of PPO is: (i) to increase the gas permeability, and to decrease the gas permeability ratio, in comparison to sulfonated PPO (SPPO), while on the other hand, (ii) to decrease the gas permeability, and to increase the gas permeability ratio, in comparison to brominated PPO (PPOBr). A noticeable trend in sulfonated brominated PPO membranes, when the degree of bromination is increased while the same degree of sulfonation is maintained, is the occurrence of a minimum in both gas permeability and diffusivity at 37.4 % degree of bromination. All these effects produced by sulfonation of PPO, or bromination of PPO, or simultaneous sulfonation and bromination of PPO, are believed to be the direct result of the manipulation in the polymer backbone stiffness, membranes density, and membranes packing and free volume fraction. (c) 2005 Elsevier B.V. All rights reserved.