BACKGROUND: Olive mill wastewater (OMW) constitutes a very strong agro-industrial wastewater posing severe environmental threats in olive oil producing countries. The main objective of this study was to treat olive mill wastewater by electrochemical oxidation. The variables studied included the type and concentration of electrolyte solutions, voltage and time applied. RESULTS: The electrolyte type and concentration significantly affected the degradation efficiency of the electrochemical oxidation. Optimal conditions for NaCl concentration were 3% (w/v) and 16 V. At these conditions chemical oxygen demand (COD) removal reached 70.8% after 8 h of electrochemical treatment, while color and turbidity were completely removed after short periods of treatment. However, bio-assays indicated that the ecotoxicity of the treated wastewater remained unchanged, possibly due to the formation of chlorinated by-products. Na2SO4 did not demonstrate sufficient efficiency. The simultaneous use of FeCl3 and NaCl contributed to electro-coagulation of OMW. After settlement, two separate phases were formed: the supernatant phase and the settled solids. Under optimal conditions (2% Na2SO4 + 1% FeCl3; 24 V), the removal efficiency of COD reached 85.5% at the supernatant phase. CONCLUSION: NaCl was an effective electrolyte for OMW treatment. The electro-coagulation process was also a successful process, but as in the case of NaCl the remaining acute toxicity of treated OMW was high. (c) 2007 Society of Chemical Industry.