Integrally skinned asymmetric flat sheet membranes were prepared from poly(2,6-dimethyl 1,4-phenylene oxide)(PPO) for CO2-CH4 separation. Various experiments were carried out to identify PPO membranes, which have good mechanical strength and gas separation abilities. Membrane strength and selectivity depend on the interplay of the rate of precipitation and the rate of crystallization of the PPO. The effects of major variables involved in the membrane formation and performance, including the concentration of the polymer, solvent, and additive, the casting thickness, the evaporation time before gelation, and the temperature of the polymer solution, were investigated. Factorial design experiments were carried out to identify the factor effects. The membrane performance was modelled and optimized to approach preset values for high CO2 permeance and a high CO2: CH4 permeance ratio. Membranes were prepared based on the optimum conditions identified by the model. Essentially, defect-free membranes were prepared at these conditions, which resulted in a pure gas permeance of 9.2 x 10(-9) mol/m(2) s Pa for CO2 and a permeance ratio of 19.2 for CO2: CH4.