A cellulose acetate matrix was irradiated in the dose range 0-250 kGy by gamma radiation from a Co-60 source under an argon atmosphere; positron lifetime experiments were performed under the same atmosphere at room temperature. The results reveal a dominant intermediate component tau(2) (similar to 430 ps) attributable to a crystalline macrocellulosic domain. Important changes are shown by the long-lived pick-off component tau(3) due to the degradation in the polymer matrix.