Poly(3-hydroxy butyrate-co-3-hydroxy valerate) containing 11 mol% 3-hydroxy valerate (HV) units has been subjected to gamma-irradiation in vacuo at 293 K to various doses up to 30 kGy. Radical yields estimated by ESR at 293 K increased linearly with dose up to 5 kGy. This dependence, in conjunction with the corresponding linear one measured for a gamma-irradiated standard (glycine), yielded a value of G(radical) = 1.7 +/- 0.5 (h eV)(-1) for the polyester, which is discussed in relation to values reported for microbial polyesters of different contents of HV. The radical yield remained constant over a measurement period of 200 min, whereas, after the ESR tube was opened to the atmosphere, the radical yield fell by 80% over the same time, indicating quenching by oxygen. The production of peroxidised polymer on continuous gamma-irradiation in air was confirmed qualitatively.