The aim of this study was to determine the influence of wastes physical parameters on their biodegradability. Humidity, particle size and mixture ratio were identified as three physical parameters likely to influence the accessible biodegradable potential of wastes co-composted with a bulking agent. Mixtures of sludge and bulking agent were studied in a respirometric device leading to oxygen consumption rates. The respirometric profiles were modelled to estimate the wastes biodegradation kinetics. The results proved that the biodegradable potential of the mixtures varied as a function of the water content, the size class of the bulking agent and the mixture ratio. Moisture presented the greater impact on the total amount of oxygen consumed. The kinetics of the biodegradation reaction by composting were also determined and are influenced by two main factors: humidity and particle size.