The gas separation properties of new aromatic polyoxadiazole (POD) and polytriazole (PT) membranes have been investigated. A wide range of permeability and selectivity was found for polymers with 4,4’-diphenyl-ether unit (POD-DPE and PT-DPE), 4,4’(2,2’-diphenyl)hexafluorisopropane (POD-HF), 1,1’,3-trimethyl-3-phenylindane (POD-PIDA) and 5-tert-butylisophthalic (POD-TBI) units incorporated into the backbone. The differences found in permeability and selectivity, as well as the dual sorption parameters of POD-PIDA and POD-DPE polyoxadiazoles are discussed in terms of their free volume determined by group contribution.