The present work studies the changes occurring in organic matter, phenols and biotoxicity on composting olive mill wastewater with barley straw. The total organic matter decreased, a drop of 25% was reached after the stabilization phase and 52% at the end of the maturation phase. Degradation of the phenols reached 54% and 95%, respectively, after these periods. The toxicity of the water extract, evaluated by the Photobacterium phosphoreum fluorescence, decreased to disappear after only 2 months of composting. This trend was confirmed by the tight correlation between the physico-chemical and toxicity parameters, indicating that the degradation of organic matter leads to a strong reduction of the C/N ratio and of toxicity. The results obtained indicate a normal process of humification occurring during the composting of the formerly highly toxic olive mill wastewater-straw mixture and resulting in a product, which has good agricultural properties as organic fertilizer. (c) 2006 Elsevier B.V. All rights reserved.