Phenol formation by gamma radiolysis of an aqueous benzene solution containing sodium hypochlorite is reported. The phenol formation in a benzene solution containing sodium hypochlorite irradiated with Co-60 gamma-rays is about six times higher than that without sodium hypochlorite. Ten micromolar sodium hypochlorite enhanced the formation of phenol up to a total dose of 6 Gy. Above 6 Gy in solutions containing sodium hypochlorite, the rate of phenol yield sharply decreased and was essentially the same as that without sodium hypochlorite. The yield of phenol with sodium hypochlorite is 0.89 mu mol J(-1) and is larger than the sum of yield for the radicals and reactive oxygen species by water radiolysis such as (OH)-O-center dot, e(-), H, H-2, and H2O2. The formation of phenol with sodium hypochlorite was reduced by NaCl. Results suggest that the radiolytic formation of phenol in a benzene solution with sodium hypochlorite relates to the reaction process involving chlorine atoms. Sodium hypochlorite can be applied as a sensitizer for a benzene chemical dosimetry system. The lower limit of dose detection calculated from the detection limit of phenol and the G value of phenol was estimated to be 1 x 10(-3) Gy.