Solution cast membranes of poly(3-dodecylthiophene) (PDDT) were studied for the room temperature separation of N-2, O-2, and CO2. A procedure for fabricating reproducible, smooth, uniformly thick (similar to 35 mu m), defect-free membranes is described Permeability values were measured for as-cast PDDT membranes (P-N2 = 9.4, P-O2 = 20.2, P-CO2 = 88.2 Barrers) and selectivity values were calculated (alpha(O2)/N-2 = 2.2, alpha(CO2/N2) = 9.4). Chemically induced oxidation (similar to 23%) with SbCl5 resulted in a decrease in permeability (P-N2 = 3.5, P-O2 = 10.5, P-CO2 = 48.5 Barrers) and a corresponding increase in permselectivity (alpha(O2/N2) = 3.0, alpha(CO2/N2) = 14.0). Reduction of the oxidized membrane with hydrazine partially reversed these trends (P-N2 = 5.4, P-O2 = 15.1, P-CO2 = 62.9 Barrers, alpha(O2/N2) = 2.8, alpha(CO2/N2) = 11.6). The chemical compositions of as-cast, oxidized, and hydrazine-treated PDDT membranes were determined using elemental analysis and energy dispersive X-ray spectrometry. Membrane microstructure was investigated by optical microscopy. TappingMode(TM) atomic force microscopy and scanning electron microscopy. The composition and microscopy results were correlated with changes in gas-transport properties.