This study evaluated the effectiveness of ultraviolet (UV) radiation on the photodegradation of the herbicides amitrole (AMT), clopyralid (CLP), fluroxypyr (FLX), and diuron (DRN), investigating the influence of operational variables (initial herbicide concentration and pH) and chemical nature of water (ultrapure, tap, and wastewater) on these processes. We also followed the time course of total organic carbon (TOC) during photodegradation of the herbicides and their photodegradation byproducts. The quantum yield ranged between 1.27x 10(-2) mol Einstein(-1) (DRN) and 6.77 x 10(-2) mol Einstein(-1) (CLP), rising with the decrease in HOMO-LUMO energy gap of the triplet states of the herbicide molecules. Photodegradation rate constants ranged between 0.013 min(-1) (AMT) and 0.220 min(-1) (DRN). Herbicide degradation rates followed the order DRN > FLX >> CLP AMT. Comparison of the time course of herbicide photodegradation with that of the corresponding TOC values showed that the four herbicides were initially degraded to lower molecular weight byproducts that then continued to mineralize, degrading to CO2, in the presence of UV radiation. The photodegradation process is highly dependent on the medium pH, generally being favored at pH values at which herbicides are in ionic form. With regard to water type, the herbicide photodegradation rate decreased in the order ultrapure > tap > wastewater; thus, rate constants for DRN were 0.220 min(-1), 0.194 min(-1), and 0.114 min(-1), respectively. The organic and inorganic byproducts (NO3-, NH4+, Cl-) generated by herbicide photodegradation were determined; the results indicate that the degradation mechanism involves substitution of Cl of the aromatic ring with Ha radicals and the formation of byproducts containing OH in the ring. (C) 2014 Elsevier B.V. All rights reserved.