Solar photo-Fenton and solar photo-Fenton-ferrioxalate processes using a compound parabolic collector (CPC) were applied to the degradation of Reactive Blue 4 (RB4) solutions, proving to be an efficient method. Multivariate experimental design (including the following variables: pH and initial concentrations of Fe(H), oxalic acid, H2O2 and RB4) was used. The efficiency of photocatalytic degradation was determined from the analysis of the following parameters: color removal, total organic carbon (TOC) and chemical oxygen demand (COD). The decoloration rate pseudoconstant was calculated as a function of the accumulated solar energy received by the water solution. Experimental data were fitted using neural networks (NNs) which reproduce experimental data within 86% of confidence and allows the simulation of the process for any value of parameters in the experimental range studied. Analysis of dissolved H2O2 during reaction also helps to explain the scavenger effect and the dye mineralization grade. The solar photo-Fenton-ferrioxalate process increases degradation rate of RB4 since ferrioxalate complexes absorb strongly and a higher portion of the solar spectrum can be used. Although the addition of oxalic acid increases operating costs, it improves the process and it also helps to reduce pH in solution, decreasing charges derived from this operation. Under the optimum conditions, ([H2O2] = 120 ppm (in two additions), [Fe(Il)] = 7 ppm, [(COOH)(2)] = 10 ppm, pH 2.5), color and COD were completely removed whereas TOC was reduced up to 66%. (C) 2007 Elsevier B.V. All rights reserved.