A new hybrid gas separation membrane was prepared from poly(2,6-dimethyl-1,4-phenylene oxide) modified with graft copolyimide with side poly(methyl methacrylate) chains. The changes in the membrane structure on introducing up to 15 wt % modifier were evaluated by atomic force microscopy and density measurements. The microphase separation in modified polyphenylene oxide films was demonstrated. Introduction of graft copolyimide leads to an increase in the density of the hybrid films. The gas transport properties of the membranes were evaluated for H₂, CO₂, O₂, O₄, and N₂. Introduction of up to 10 wt % modifier does not noticeably alter the permeability of the hybrid membranes to all the gases but increases the selectivity in gas separation.