Poly(2,6-dimethyl-1,4-phenylene oxide), PDMPO, poly(2,6-diphenyl-1,4-phenylene oxide), PDPPO, as well as their copolymers of different compositions, having both random and block structures, have been synthesized and characterized by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, and gel permeation chromatography. Solution-cast films were prepared from all synthesized polymers using chloroform as a solvent. The thermal properties of the resulting films were characterized by differential thermal analysis and differential scanning calorimetry, whereas their morphology was investigated using X-ray diffraction. Ultimately, the potential of the synthesized polymers for gas separation was studied by examining gas permeation properties of the respective thin films in single gas permeation tests involving N-2, O-2, CH4, and CO2. In general, the O-2 and CO2 perme-ability coefficients decrease with the PDPPO content. However, the largest drop in the permeability coefficients occurs between PDMPO and a copolymer having the lowest PDPPO content, and the permeability coefficients PDPPO are comparable or even lower than the permeability coefficients of the copolymers having the largest PDDPO content. On the basis of combination of the permeability coefficients and their ratios for CO2/CH4 and O-2/ N-2, random copolymers appear to be a better candidate for gas separation membranes than their block counterparts. (C) 2007 Wiley Periodicals, Inc.