Changes occurring in the chemical structure of fulvic acids extracted from composted sewage sludge at different times of treatment were investigated by various chemical methods (elemental analysis, Fourier transform infrared and carbon-13 nuclear magnetic resonance). The starting material was composed of aromatic structures, carbohydrates, polyalcohol or polyacid alkyl side-chains. During the stabilisation phase of composting, microbial metabolism seems to begin by the decomposition of aliphatic alcohols or acids shown by a decrease of absorbance around 1136 cm(-1), a resonance in C-alkyl and carboxyl areas. In parallel, structures with anomeric, cetal or ketal carbons and aromatic compounds around 102, 130-160 ppm respectively, were preserved or yielded from microbial decomposition. At the end of composting, the increase of resonance around 32 and 174 ppm in the NMR spectra and the increase of absorbance around 1142 cm(-1) in the FTIR spectra suggest the presence of ester/ether linkages and neoformation of alkyl ethers/esters. The latter structures could be considered as new constituent moieties incorporated during humification, rather than compounds somehow trapped in the fulvic acid structures. The end product exhibits more aromatic and polycondensed structures, which are chemically and biologically more stable and less reactive than the starting material. (c) 2004 Elsevier Ltd. All rights reserved.