A static respirometer was built to determine the Respirometric Index (RI) of composting samples at different temperatures. Composted materials were dewatered sludge (DS) and dewatered anaerobically digested sludge (ADS) obtained from a wastewater treatment plant. Respirometric indexes were obtained at a fixed value of 37 ºC (RI 37 ) and at the in situ temperature of the reactor at the moment of sampling (RI T ). Results indicated that both indexes correlated well with temperature evolution in the composter.However, RI T were more representative of the metabolic activity in the reactor and were also more sensitive to temperature and composition variations of the composting material. Nevertheless, when process temperature felt below optimum mesophilic conditions, RI T were useful for the monitoring but not for determining the actual stability of the material. Moreover, the importance of the units used to express RI was also analysed during the composting of sludge from the paper industry, a highly compostable residue. RI expressed on dry matter basis indicated that this was a stable