Aerobic digestion of olive mill waster-water (OMW) has been conducted under various medium conditions to determine the best treatment involving good stabilisation and maturity of these residues. Analysis by various chemical methods of elemental analysis, FTIR and C-13 NMR spectroscopies show the high content of raw olive mill waster-water of the less condensed structures of phenols, organic acids, alcohol, fatty acids and simple sugars. Aerobic digestion involved an humification process through two mechanisms - biodegradation and polycondensation. These mechanisms were highly influenced by medium conditions. In the cases of natural acid pH, either in the presence of soil microflora or the yeast Saccharomyces cerevisiae, aerobic digestion has been less developed due to the antimicrobial effect of free phenol. Neutralization of pH enhances the development of microbial activity and the humification process seems greatly influenced by means of neutralization. In the case of neutralization by lime, the intense oxidation of organic compounds occurred and humification involved polyphenol condensation. While, in the case of neutralization by phosphate, more oxidation of sugars has been observed, and polycondensation in contrast developed through N-linkage. Accordingly, treatment of olive mill waste-waters by soil micro-flora with neutralization of pH by phosphate could be considered the best treatment that allows good stabilisation of organic matter and high preservation of nitrogen in humic form. This treatment corrects further the deficiency of the two elements phosphorus and nitrogen. (c) 2004 Elsevier Ltd. All rights reserved.