The influence of various reaction parameters on herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) removal were examined in the photo/ferrioxalate/H₂O₂ system, with regard to: (1) sulfate, phosphate, and OH scavenger, as solution constituent; and (2) light intensity, ferrioxalate, H₂O₂, and oxalate concentration, as operating parameter. In terms of 2,4-D removal, the photo/ferrioxalate/H₂O₂ system has always been superior to the photo/Ferric ion/H₂O₂ system, despite the high presence of anions (sulfate 100 mM, phosphate 1 mM) or OH scavenger. Not only the rate of 2,4-D removal, but also the decomposition rate of H₂O₂ and oxalate proportionally increase with light intensity. The ferrioxalate concentration determines the light absorption fraction, and thus, controls the rates of 2,4-D removal, and the decomposition of H₂O₂ and oxalate, are predicted from kinetic formulations. The optimal concentration of H₂O₂ and oxalate, according to the extent of the OH scavenging reaction with these reagents, has been demonstrated for 2,4-D removal. It was found that an increasing oxalate concentration, which bears the burden of increased dissolved organic carbon (DOC), does not occur. This is because its decomposition, as a result of the photochemical reduction of the ferric oxalate complex, results in a decrease of the equivalent DOC. The importance of the key reaction factors to be considered, when applying this system to real wastewater treatment, is also discussed.