The free‐radical density produced in polymethylmethacrylate under x‐ray and &ggr;‐ray irradiation has been measured as a function of the amount and kind of organic additive substances present in the polymer. The ratio of radical density produced in the pure polymer by a given radiation dose to that produced in polymers containing additives exhibits a strong dependence upon radiation dose, unlike the similarly defined ratio for the protection against radiation damage by polymer chain scission. The time rate of decay of free‐radical density was also found to depend upon the additives present. The existence of a radiation‐dependent free‐radical annihilation process is postulated.