Transport of CH4, C2H4 and C2H6 in poly(phenylene oxides) membranes at low pressures has been studied. The relation between the free volume and permeability of the polymers was analyzed in terms of the ＇dual-sorption＇ model. The accessible free volume of the polymers was estimated assuming the density of a sorbed fluid is equal to the density of the corresponding liquid. Transient separation of the three-component mixture CH4/C2H4/C2H6 was studied.