An experimental study based on ferrioxalate-assisted solar photo-Fenton (SPFox) process shows how non-biodegradable azo dye Orange II (OII) solutions degradation can be enhanced or slowed down by continuous addition of hydrogen peroxide and air injection depending on operation conditions. The decoloration and mineralization of dye solution has been carried out in a solar Compound Parabolic Collector (CPC). An optimization study was done by using Multivariate Experimental Design including the following variables: flow rate of H2O2, air flow rate, pH and initial concentrations of Fe(II) and oxalic acid. The efficiency of photocatalytic degradation was determined from the analysis of color and Total Organic Carbon (TOC). Experimental data were fitted using neural networks (NNs) which allow the simulation of the process for any value of variables in the studied experimental range. The results reveal that the continuous addition of H2O2 improves the photocatalytic efficiency since the scavenger effect of peroxide is minimized. On the other hand, this system permits the use of a ferrous concentration below the discharge legal limit (2 ppm) being bubbling of air not necessary in that conditions. In addition, oxalic acid can be used to pH adjustment, reducing the operation costs of Fe removal, chemicals and electric power. Under the optimal conditions, 100% decoloration of dye solution can be reached by using both processes (SPFox with H2O2 addition at the beginning or along the reaction) but with different reaction rates. However, the efficiency of TOC removal was higher in the SPFox process with continuous addition of H2O2 (95% TOC removal in SPFox system with continuous addition of peroxide versus 80% TOC removal in SPFox system when peroxide is added at the beginning of the reaction). Molecular and/or radical reaction pathway was Studied by conducting the reaction in the presence and absence of tert-butylalcohol. (C) 2009 Published by Elsevier B.V,