Rearrangements of the porous structure of the glassy polyphenylene oxide copolymer occurring in the course of its swelling in the atmosphere of several gases and the subsequent relaxations were investigated using the low temperature nitrogen adsorption technique. The values of porosity (epsilon) and BET surface area (S-BET) were used to quantify the copolymer's free volume accessible to N-2 molecules at 77 K. The magnitude and the rate of changes of the copolymer's free volume invoked by its conditioning in vacuum, N-2, O-2, CH4, CO2, and C3H6, were analysed in terms of s and SBET. These characteristics of the polymer structure were found to depend upon the sorbate nature, its pressure. exposure time and temperature. Swelling of the copolymer that occurs in certain atmospheres results in the expansion of the free volume, which is reflected in the increased values of epsilon and S-BET. In accordance with the magnitude of the copolymer swelling under experimental conditions employed in this study, the gases can be qualitatively ranked as follows: C3H6 > CO2 > CH4 > O-2 approximate to N-2. The swollen polymer, being placed in the environment of a lower swelling ability, experiences structural relaxations. The relaxations result in a decrease of the free volume, which is reflected by a decrease in the values of epsilon and S-BET. Gas permeability of glassy polymer membranes is directly related to the conditioning history of a polymer: the state of the glassy polymer characterised by the increased values of porosity and BET surface area corresponds to the more permeable state of a membrane.