[en] The behavior of the copolymer poly(ethylene-co-vinyl acetate), with different amounts of vinyl acetate (VA) (2, 4.5, 9 and 19 w/w %, respectively), has been studied by performing positron lifetime measurements as a function of temperature. An overall reduction of the ortho-positronium formation probability I₃ with increasing VA content is observed. The copolymers with 2, 4.5 and 9 w/w % VA show a V-shaped I₃ evolution with temperature, caused by positron irradiation induced ionization, whereas the copolymer with 19 w/w % VA has become practically inert to self-irradiation effects.