Pre-treatment of olive mill wastewater (OMW) by Fenton Oxidation with zero-valent iron and hydrogen peroxide was investigated to improve phenolic compounds degradation and the chemical oxygen demand (COD) removal. Experimental procedure is performed with diluted OMW with COD 19 g/L and pH 5.2. The application of zero-valent Fe/H2O2 procedure allows high removal efficiency of pollutants from OMW. The optimal experimental conditions were found to be continuous presence of iron metal, acidic pH (2-4) and 1 M hydrogen peroxide solution. The experimental results show that the removal of 1 g of COD need 0.06 M of H2O2. At pH 1, the maximum COD removal (78%) is achieved after 1 h. Therefore, with a pH value within 2 and 4 the maximum COD removal reached 92%. Phenolic compounds are identified in treated and untreated OMW by gas chromatography coupled to mass spectrometry (GC-MS). The result shows a total degradation of phenolic compounds and an increasing biodegradability of treated OMW. (C) 2009 Elsevier B.V. All rights reserved.