Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) powder and the dense membranes prepared from it by using 1,1,2-trichloroethylene (TCE) as a solvent were characterized by electron spin resonance (ESR) technique. It was observed that PPO and membranes have free radicals. However, in the membrane the number of spins/g was higher than in the PPO powder regardless of the temperature used for the preparation of the membrane. PPO and the membranes all have electron donating sites when studied by using tricyanoethylene (TCNE) solution in benzene. Hamiltonian parameters and g values for all radicals were measured. It was noticed that the permeation rate of CH4 was less than the permeation rate of CO2. On the other hand CH4 have higher affinity for the formation of radicals in the membrane than CO2 under similar conditions. The electron donating sites were maximum in the membrane prepared at room temperature.