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 25, 35, 45, 55 and 65 degreesC evaporation temperature. The effect of temperature used for evaporation of solvent on the characteristics of the membrane was studied, by using electron spin resonance, atomic force microscopy and gas permeation rate. The ESR signal from the spin probe in the membrane, the membrane surface morphology and the gas permeation through the membrane were all affected by evaporation temperature. The membrane becomes denser according to the ESR study; the surface becomes rough according to atomic force microscopy and, the permeation of CO2, CH4, O-2 and N-2 decreases according to gas permeation experiments as the evaporation temperature increases. However, no significant change was observed on the permeability ratio for gas pairs CO2/CH4 and O-2/N-2. A possible mechanism has been discussed. (C) 2003 Elsevier B.V. All rights reserved.