In wastewater treatment using the Fenton and photofenton processes, pH is one of the critical operating parameters, due to the fact that the Fenton reaction can work only under acidic pH conditions. It is hoped that Ferric iron complexed with oxalate (Fe(III)-oxalate; ferrioxalate) will provide an alternative to the traditional Fenton process with its limited range of pH conditions, since its high solubility in aqueous media can broaden the available pH range of the Fenton reaction up to the near neutral pH regime. In this study, we investigated the pH dependency of OH production in the photo/ferrioxalate system, in the presence and absence of externally supplied H 2O 2, where 2,4-D was used as the probe compound for OH production at a wide range of pH values (1.2–7.4). In the absence of externally supplied H 2O 2, the 2,4-D degradation was considerably enhanced with increasing pH, whereas it was reduced with increasing pH in the presence of an excess amount of H 2O 2. These variations in the degradation of 2,4-D were thus found to be precisely related to the formation of H 2O 2, a factor to which little attention was paid in previous studies. In the absence of H 2O 2 addition, the in situ formation of H 2O 2 is facilitated with increasing pH by the reaction of Fe(II) with O 2 - , which increases with pH, augmenting the production of OH and thereby leading to the faster degradation of 2,4-D. This same reaction can also provide an explanation for the opposite pH dependence of 2,4-D degradation in the presence of H 2O 2.