A central composite rotatable design and response surface methodology were used to optimize the experimental variables of the electro-Fenton (EF) and solar photoelectro-Fenton (SPEF) degradations of 2.5 L of sulfanilic acid solutions in 0.05 M Na2SO4. Electrolyses were performed with a pre-pilot flow plant containing a Pt/air diffusion reactor generating H2O2. In SPEF, it was coupled with a solar photoreactor under an UV irradiation intensity of ca. 31 Wm(-2). Optimum variables of 100 mA cm(-2), 0.5 mM Fe2+ and pH 4.0 were determined after 240 min of EF and 120 min of SPEF. Under these conditions, EF gave 47% of mineralization, whereas SPEF was much more powerful yielding 76% mineralization with 275 kWh kg(-1) total organic carbon (TOC) energy consumption and 52% current efficiency. Sulfanilic acid decayed at similar rate in both treatments following a pseudo-first-order kinetics. The final solution treated by EF contained a stable mixture of tartaric, acetic, oxalic and oxamic acids, which form Fe(III) complexes that are not attacked by hydroxyl radicals formed from H2O2 and added Fe2+. The quick photolysis of these complexes by UV light of sunlight explains the higher oxidation power of SPEF. NH4+ was the main inorganic nitrogen ion released in both processes. (C) 2012 Elsevier B.V. All rights reserved.