Dense (homogeneous) membranes were prepared from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) by using 1,1,2-trichloroethylene as a solvent at different solvent evaporation temperatures (22, 4 and -10°C). The effect of temperature used during evaporation of solvent on the characteristics of the membrane was studied by using electron spin resonance, atomic force microscopy and gas permeation rate. The morphology of the surfaces of the membrane, the shape of spin probe in the membrane, and the selectivity of gases depend on the temperature of evaporation of solvent. The permeation rate of CO 2 increased with the decrease in the temperature used for the preparation of the membrane. However methane permeation rate increased in the membrane prepared at -10°C. It is suggested that Langmuir sites could be favorable for the CH 4 permeation.