The enhanced effect of gamma irradiation with hydrogen peroxide (H2O2) for alachlor degradation in an aqueous solution was first investigated in this study. The combination of gamma irradiation and H2O2 led to an enhanced effect, which remarkably increased the degradation efficiency of alachlor and the total organic carbon (TOC) removal. At a dose of 200 Gy, the degradation degree of the alachlor solution reached 81.7 and 99.2% under H2O2 concentrations of 0 and 0.1 mu M, respectively. In addition, the TOC removal efficiencies of the alachlor under initial H2O2 concentrations of 0.0.5 and 1.0 mu M were 59.5, 74.8 and 83.8%, respectively, at an absorbed dose of 20 kGy. However, for higher H2O2 concentrations (greater than 1 mu M), the alachlor degradation was reduced because (OH)-O-center dot radicals were scavenged by the H2O2. The biodegradability of alachlor solutions prior to and after treatment by gamma irradiation was also assessed using the Closed Bottle Test (CBT). The results showed enhanced biodegradability of alachlor with increasing absorbed doses. (C) 2010 Elsevier B.V. All rights reserved.