Miscible blends of polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) were prepared by a solution casting method to examine the permeation characteristics for oxygen and nitrogen. Gas permeation characteristics of PS/PPO membranes prepared using two different methods, slowly cooled and quenched, after being treated at various annealing temperatures were examined with respect to the controlled free volume. A variety of compositions of PS/PPO membranes provided varying permeability coefficients and ideal separation factors. This indicates that the present miscible blends gave rise to a decrease in the free volume by the physical interaction between two polymer components. For the slowly cooled PS/ PPO membranes, the permeability coefficient decreased but the ideal separation factor increased with decreasing annealing temperatures due to the fast macromolecular relaxation or the efficient packing effect of macromolecular chains. On the other hand, the permeability coefficient of the quenched PS/PPO membranes increased with increasing annealing temperatures, as the quenching from higher annealing temperatures led to the existence of more free volume.