In this study, the potential of Fe(II)-catalyzed activation of Oxone process with UV irradiation (FOU) for the degradation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aqueous solution was explored and compared with the Fe(II)/Oxone (FO) process. Experimental results show that the FO process was dramatically promoted upon the introduction of UV irradiation. As a result, the role of UV irradiation was elucidated in-depth by comparing the real-time of [Fe(II)] between the FO process and the FOU process. It was found that a beneficial Fe(III)/Fe(II) catalytic cycle is established in the presence of UV irradiation, thereby leading to the accelerated regeneration of Fe(ll). Additionally, 2.4,5-T decay by the FOU process under various [Oxone] and [2,4,5-T] was also examined. Furthermore, the decay pathways for the transformation of 2,4,5-1 by UV alone, Oxone/UV. and FOU processes were proposed by using LC-ESI/MS analysis. Distinct differences of intermediate distributions were observed among the three processes with or without the involvement of radicals ((OH)-O-center dot and SO4(center dot-)). Besides, the efficiencies of various processes (i.e., UV alone, Oxone/UV and FOU) were further examined in terms of mineralization and CI- ion accumulation. (C) 2012 Elsevier B.V. All rights reserved.