The one-electron oxidation of phenanthrenequinone (PhQ) in aqueous solutions was investigated by pulse radiolysis using hydroxyl radical and sulphate radical as oxidants. Spectral and kinetic properties of the transients formed due to the reaction of PhQ with the oxidants were obtained. The reaction of hydroxyl radical with PhQ results in an initial species with an absorption spectrum centered at 370nm and a weak broad band in the long wavelength regions at neutral pH. The bimolecular rate constant for the formation of initial species was determined to be (1.5±0.2)×10,10,dm,3, •mol,-1, •s,-1,. The absorption in the short wavelength region decays with a concomitant increase in the longer wavelength region resulting in a subsequent species with absorption maximum at 400nm and a broad band centered at 500nm. The absorption in visible region increases with increasing pH. Similar transient absorption was obtained at pH 9.9. At pH 4.0, the initial spectrum also shows absorption maximum at 370nm. However, the subsequent absorption spectrum shows a maximum at a shorter wavelength than those found at pHs 7.1 and 9.9. The absorption increase in visible region was not seen at low pH. The transient spectrum obtained at neutral pH by the reaction of sulphate radical with PhQ are similar to the subsequent spectra for the reaction of hydroxyl radical with PhQ showing two bands centered at 400 nm and 500 nm. The bimolecular rate constant was estimated at (4.0±0.6)×10,9,dm,3, •mol,-1, •s,-1,. On the basis of spectral similarity and kinetic analysis, a possible mechanism for the oxidation of PhQ by both hydroxyl radical and sulphate radical is given.