One-electron reduction of 1,10-phenanthroline-5, 6-quinone (PQ)in aqueous solutions was studied by ,60,Co γ-radiolysis using isopropanol free radical as a reductant. The UV/vis absorption spectrum obtained after irradiation at pH9 showed a new broad band centered at 490 nm. Different absorption spectra were obtained at pH≈7 and ≈4. Below pH 7, the increase in the absorption around 490nm was much small and decreased with decreasing pH. These observations agree with that in pulse radiolysis experiments where a residual absorption at about 490nm remained constant for several milliseconds,and this residual absorption increased with increasing pH,[1~3],. It is suggested that the absorption in the visible region observed after the γ-irradiation and millisecond after pulse radiolysis is caused by the same species, a final product. MS analysis indicates that a final product with more mass number that PQ was produced after γ-irradiation at pH 3.3. This final product was assumed to be PQH,2, produced by the disproportionation of PQ semiquinone neutral radical.