The study evaluated the effect of Fenton's oxidation on the particle size distribution (PSD) of significant parameters reflecting the organic carbon content of olive oil mill wastewater (OMW). The organic carbon content of the studied OMW was characterized by a COD level of around 40,000 mg/L, with 13,500 mg/L of TOC and 1670 mg/L of total phenols. The corresponding antioxidant activity (AOA) was determined as 33,400 mg/L. PSD of the selected organic carbon parameters was investigated using a sequential filtration/ultrafiltration procedure. COD fractionation based on PSD revealed two major components, a soluble fraction below 2 nm and a particulate fraction above 1600 nm representing 49% and 20% of the total COD, respectively. The remaining COD was distributed in the colloidal and supracolloidal zones. The PSD of TOC, total phenols and AOA exhibited similar profiles with peaks at the two ends of the studied size range. Overall COD removals achieved via Fenton's oxidation both at pH = 3.0 and pH = 4.6 (the original pH of the OMW) remained in the range of 40-50%. As anticipated, the effect of Fenton's treatment was more pronounced in the soluble size range. Fenton's oxidation at pH = 3.0 resulted in 46% and 63% removals for total phenols and AOA, respectively. The results obtained indicated that Fenton's process could only be useful as an alternative preliminary treatment option of the required full treatment scheme that could involve a sequence of filtration, oxidation and/or biological treatment steps. (C) 2009 Elsevier Ltd. All rights reserved.