The aim of this study was to follow the decline in the biodegradability and the resulting organic matter stabilisation during composting treatment, by means of a respirometric device. A mixture of wastewater sludge and pine barks was composted over 21 days in a 300 L stainless-steel insulated pilot reactor supplied with compressed air. Samples were taken after 0, 5, 12 and 21 days of composting and were studied by respirometry. The respirometric profiles were modelled to estimate the amount of biodegradable organic matter and its easily and slowly biodegradable fractions. These results proved the decline in biodegradability of the organic matter with composting time as the total biodegradable organic matter content decreased between 0 and 21 days of composting. The comparison of this parameter between each sample revealed the absence of significant matter biodegradation between 12 and 21 days of composting. This information could have helped modify composting management to complete biodegradation of the material. In the same way, the estimation of oxygen that could be consumed by means of respirometry on initial mixture could help adjust the composting time or other parameters such as airflow rate. In order to study more closely the biodegradation processes, the respirometric experiments on these 4 samples were associated with biochemical characterisations of global materials and of their water-soluble and non water-soluble fractions. The chemical oxygen demand measurements did not reveal a tendency for the organic matter evolution, this as well on global samples as on their soluble and non-soluble fractions. In revenge, the biological oxygen demand measured by liquid respirometry on aqueous fractions confirmed the decrease with composting time in easily biodegradable organic matter. This was already displayed by modelling results. Thus liquid respirometry emerged as a reliable tool, associated with solid respirometry, to observed the organic matter stabilisation during composting.