The degradation mechanism of p-nitrophenol (p-NP) exposed to 254 nm UV light was studied in the presence and the absence of oxygen respectively via both steady-state photolysis and time-resolved laser flash photolysis (LFP) experiments. It has been confirmed that p-NP can be photo-ionized to produce its radical cation (p-NP+center dot) and hydrated electron (e(aq)(-)) with a quantum yield of 0.52. In neutral solution p-NP+center dot will be quickly deprotonated to form its phenoxyl radical (p-NP center dot) which will react with oxygen to promote the breakage of benzene ring of p-NP. The degradation efficiency of p-NP exposed to 254 nm UV is as low as commonly reported. However, oxygen could improve the photo-degradation efficiency, which is due to the reaction of oxygen with p-NP center dot. The reaction between oxygen and p-NP center dot has been experimentally confirmed both in LFP and in pulse radiolysis. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.