The catalytic degradation of Orange II in a ferrioxalate-assisted photo-Fenton process with combined solar and artificial ultraviolet light sources and continuous addition of H2O2 was investigated. The reaction was carried out in a pilot plant consisting of a compound parabolic collector (CPC) solar reactor in series with a UV-A/C reactor. An optimization study was done using a multivariate experimental design including the following variables: pH. H2O2 flow rate. UV-lamp exposure time, average temperature, average solar power and initial concentrations of Fe(II) and oxalic acid. The photocatalytic degradation efficiency was determined by the analysis of color and total organic carbon (TOC) removal. Under the optimum conditions. TOC removal increased to 99% in only 45 min, and this system permitted the use of a low ferrous concentration of only 2 mg L-1. In addition, oxalic acid was used for pH adjustment. Thus, the operating costs of Fe removal, chemicals and electric power were reduced. Artificial UV-A/C light can be used either to increase the efficiency of the single-solar process or as an alternative to solar CPC on cloudy days. The overall rate constant was split into three components: direct oxidation by hydrogen peroxide, photolytic breakdown of dye-oxalate complexes chromophore group and oxidation by hydroxyl radicals. The influence of the Fe catalyst on the molecular and/or radical reactions was studied by conducting the reaction in the presence and absence of tert-butyl alcohol: the radical mechanism's contribution to the overall degradation increased with increasing iron levels. (C) 2009 Elsevier B.V. All rights reserved.