In order to face up to the problems of pollution linked to the management of waste, environmental policies have been developed. In Europe, these policies promote waste prevention, recycling and reuse. In this context, biological treatments as composting are encouraged because they allow an agronomic reuse of the organic matter contained in biodegradable organic wastes. Nevertheless, the development of the biological treatment largely relies on the quality of the final product and, as a consequence, on the quality of the biological activity during the treatment. The maintenance of favourable conditions (oxygen concentration, temperature and moisture) in the waste medium largely contributes to the establishment of a good aerobic biological activity and guarantees the stabilisation of the organic matter with limitation and control of odorous and greenhouse effect gaseous emissions. In large scale systems, such conditions depend on the aeration system used in the process. As the characteristics of the aeration system impose gas flows that have a large influence on heat and mass transfers, these latter have to be characterised in order to check and optimize the effectiveness of the aeration. The objective of this study was then to validate a method to diagnose the aeration system of a real scale composting process and to propose optimization ways. This work takes part in a larger project, named ESPACE and partly financed by the ANR (French national research agency), that is currently being carried out by Cemagref (French public research centre on environmental engineering), Suez-Environment and IMFT (French public research centre on fluid mechanics). The project goal is to study the influence of the wastes physical characteristics and the influence of aeration design on the whole composting process efficiency and on its gaseous emissions. The studied process was a closed reactor composting process (180 m3 rectangular box) with positive forced aeration. The air was blown from the bottom of the reactor, via two ventilation pipes. In the upper part of the reactor, air was sucked and led to a biofiltre treatment system. The treated waste was a mixture of sewage sludge and bulking agent that was composted during four weeks without turning. The gas flow characterization method consisted in injecting a gas tracer in the ventilation pipes and in analysing it in the exhaust air sucked above the solid mass. Methane was used as tracer. Retention time distribution curves were obtained and modelled. For all tracing experiments and considering the standard error on the measurement of the airflow rate, good mass balances (70 to 150 %) were obtained between the quantity of injected tracer and the quantity of tracer detected in the exhaust point of the process. These results validated the reliability of the tracing method to characterize the whole composting reactor. The global airflow pattern was characterized as a dispersed-plug flow. Along the composting time, the global dispersion coefficient of air decreased indicating that the biodegradation process tended to organise the solid medium. Moreover, the study of the mean retention time allowed determining the porous volume really aerated through the convective airflow and comparing it to the total theoretical free air space volume. All these results show that the RTD characterization is a powerful method to understand airflow patterns and to evaluate the influence of the airflow on heat and mass transfer phenomena in a large scale composting process. In the future, these results will be implemented in a composting model to test optimisation ways of the aeration system.