The waste generated in mills producing olive oil was treated in the presence and absence of natural phosphate and of various strains of micro-organisms and its degree of transformation and stability for use in agriculture was determined. Neutralisation of the effluent (treatments T2 and T3) enhanced the activity of the micro-organisms and increased the rate of degradation of the organic matter. The COD was only lowered by 58% for T1 and T3, after 20 days of treatment but fell by 86% for T2. In contrast, the reduction in the polyphenols load was 90% for T1 and 92% for T2 and T3 after 20 days. Elemental analysis showed a variable increase in the C/H and O/C atomic ratios indicating predominance of aromatic structures, especially for the T2 treatment. Infra-red spectroscopic analysis (FTIR) confirmed these results and revealed strong degradation of the aliphatic groups and enrichment in carboxylic and aromatic functions. The addition of rock phosphate seems to improve the conditions of detoxification of the olive-mill effluent as well as its stabilisation, both factors favouring recycling as soil amendment. (c) 2005 Elsevier Ltd. All rights reserved.