A static respirometer was built to determine the respiration index (RI) of composting samples. Respiration indices of different sludges were determined at 37 degrees C (RI37) and at the in situ temperature of the composter at sampling (RIT). Results indicated that both indices correlated well with temperature evolution in the composter. RIT were more representative of the metabolic activity in the reactor and more sensitive to temperature and composition variations of the composting material, but could not indicate the stability of the material at later stages of the process. Moreover, significance of RI units was shown in the composting of a highly compostable residue. According to the stability limits suggested in the literature, initial RI expressed on dry matter (DM) basis corresponds to a stable material (RI < 1 mgO(2)g DM-1 h(-1)) whereas initial RI expressed on organic matter basis (OM) corresponds to an unstable material (RI = 2.5 mg O-2 g OM-1 h(-1)). (c) 2004 Elsevier Ltd. All rights reserved.