The pre-treatment of olive mill effluents (OME) by means of coagulation-flocculation coupling various inorganic materials and organic polyelectrolytes was investigated. Tests were conducted with two different OME with chemical oxygen demand (COD) contents of 61.1 and 29.3 g/L, total suspended solids (TSS) of 36.7 and 52.7 g/L and total phenolic contents (TP) of 3.5 and 2.5 g/L, respectively. Inorganic materials such as lime, iron, magnesium and aluminum as well as four cationic and two anionic commercial poly-electrolytes were employed either alone or in various combinations and screened with respect to their efficiency in terms of TSS, TP and COD removal, the amount of sludge produced and the phytotoxicity of the resulting liquid to lettuce seeds. Coupling lime or ferrous sulphate (in the range of several g/L) with cationic poly-electrolytes (in the range of 200-300 mg/L) led to quantitative TSS removal, while COD and TP removal varied between about 10-40% and 30-80%, respectively, depending on the materials and the effluent in question; separation efficiency generally decreased with decreasing coagulant and/or flocculant concentration. To enhance organic matter degradation, iron-based coagulation was coupled with H2O2, thus simulating a Fenton reaction and this increased COD reduction to about 60%. The original, untreated OME was strongly phytotoxic to lettuce seeds even after several dilutions with water: however, phytotoxicity decreased considerably following treatment with lime and cationic poly-electrolytes; this was attributed to the removal of phenols and other phytotoxic species from the liquid phase. (c) 2005 Elsevier B.V. All rights reserved.