Fulvic acids (FAs) were isolated by a conventional procedure from two mixtures of the sludge residue obtained from olive oil mill wastewater (OMW) evaporated in open-air pond and tree cuttings (TC) at different stages of the co-composting process. The FAs were analyzed for elemental (C, H, N, S, O) and acidic functional group (carboxylic and phenolic) composition, and by ultraviolet/visible, Fourier transform infrared and fluorescence spectroscopics. At the initial stage of composting, FAs from the OMW sludge-TC mixtures were characterized by a prevalent aliphatic character, large contents of C, S-containing groups, proteinaceous materials and polysaccharide components, extended molecular heterogeneity, small O and acidic functional group contents, and small degrees of aromatic ring polycondensation, polymerization and humification. As composting proceeded, C, H and S contents, C/N ratio, and aliphaticity decreased, whereas N, O, COOH and phenolic OH contents, C/H and O/C ratios, and aromaticity increased. These results suggested that, with increasing the composting time, the chemical and structural properties of the FA components of the two OMW sludge-TC mixtures approached the characteristics typical of native soil FAs. Thus, co-composting of OMW sludge mixed with TC may represent a suitable treatment for enhancing the quality of organic matter in these materials when used as soil amendments. (c) 2006 Elsevier Ltd. All rights reserved.