In the present study, the chemical features of municipal wastewater sludges treated in two-phase separate digesters (one for acetogenesis and the other one for methanogenesis), were characterized by using chemical analysis, stable carbon isotope ratios (delta C-13), HS-SPME-GC-MS, TG-DTA analysis and DRIFT spectroscopy. The results obtained showed that Sludges from acetogenesis and methanogenesis differed from each other, as well as from influent raw sludges. Both processes exhibited a diverse chemical pattern in term of VFA and VOC. Additional variations were observed for delta C-13 values that changed from acetogenesis to methanogenesis, as a consequence of fermentation processes that led to a greater fractionation of C-12 with respect to the C-13 isotope. Similarly, the thermal profiles of acetogenesis and methanogenesis sludges greatly differed in terms of heat combustion produced. These changes were also supported by higher lipid content (probably fatty acids) in acetogenesis than in methanogenesis, as also shown by DRIFT spectroscopy. (C) 2009 Elsevier B.V. All rights reserved.