The aim of this work was to investigate the structural transformations occurring inorganic matter and their relationships with organic matter stability during the composting of two different solid olive-mill residues (SOMR). Raw materials were prepared from SOMR (compost C1) and from a mixture of SOMR and olive-mill wastewater for compost C2. Composts evolution was monitored by direct Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC) and by physicochemical parameters. Results showed that both wastes are suitable for composting and showed a strong thermophilic phase extending over the first 2 months, progressively and slowly reaching maturity after approximately 6 months. By the end of the thermophilic phase the water-soluble phenol content decreased by 93% and the pH and organic matter loss became more stable, oscillating around 8.5 and 41%, respectively. DSC and MR results showed that the biodegradation of the heterogeneous raw material is progressive and can be mainly attributed to a loss of aliphatic and peptidic structures and to an increase of the aromatic structures which essentially occurs during the maturity phase. By the end of the maturity phase, the two composts showed DSC and FTIR spectra typical of humic-like substances confirming the high degree of humification of the end products. (C) 2008 Elsevier B.V. All rights reserved.