A respirometric method was set up to study kinetics of biological reactions involved in the composting of solid organic wastes. Two types of sludge were mixed with pine bark. Oxygen consumption rates of these mixtures were monitored during 10 - 20 days, using in a 10 litre respirometric cell kept at constant temperature and moisture. Repeatability of the measurement was demonstrated. The influence on kinetics of the type of sludge and temperature were tested.