[en] Ionized states play an increasingly larger role in the radiolysis of polymethyl methacrylate as the concentration of the naphthalene additive increases. As a result, the radiation-induced losses of polymer optical fibers reached values of 1000-3000 dB/km in the wavelength range 500-1000 nm with low radiation doses of 0.01 kGy. It was established that naphthalene sensitizes the degradation of polymethyl methacrylate with formation of polyenyl macroradicals. The protective action of naphthalene in the wavelength range 4000-500 nm appears at doses above 0.5 kGy (the threshold dose for the formation of terminal macroradicals due to destruction of the polymer)