Poly(l-trimethylsilyl-l-propyne) (PTMSP)-styrene (St) grafted films were obtained by a radiochemical method. St monomer vapors were absorbed by the PTMSP, and the grafting reaction was thereafter accomplished by Co-60 gamma irradiation in a nitrogen atmosphere. The films so grafted were tested for nitrogen-oxygen separation. The performances of the membranes were studied as functions of time and percent of grafting. The PTMSP-St grafted membranes showed an increased selectivity factor and better stability in comparison with pure PTMSP. The experimental data and some SEM observations confirmed that the grafting procedure changes the PTMSP matrix structure, filling the voids responsible for the PTMSP permeability change with time.