The aim of the study was to evaluate the efficiencies of photo-Fenton (Fe(2+)) and (Fe(3+)) processes in the degradation of high-concentrations of esfenvalerate (in the form of aqueous emulsion of a commercial formulation) using a recirculation flow-through photoreactor irradiated with UV light from a 15W lamp (254 nm emission peak). The results obtained using a basic photo-Fenton (Fe(2+)) reaction (esfenvalerate 17 mg L(-1); ferrous sulphate 1 mM; hydrogen peroxide 25 mM; pH 2.5) were compared with those acquired when ferrioxalate (1, 3 or 5 mM) served as the iron source. Degradation of the active component of the commercial formulation was significantly greater, and the rate of oxidation more rapid, using a photo-Fenton (Fe(3+)) process compared with its Fe(2+) counterpart. The most efficient degradation of the insecticide (75% in 180 min) was achieved with a reaction mixture containing 5 mM ferrioxalate. However, under the same experimental conditions, degradation of pure esfenvalerate preceded much faster (99% in 60 min) and was 100% complete within 180 min reaction time. (C) 2011 Elsevier B.V. All rights reserved.