A new method, coupling a respirometer and a pycnometer, to follow the biodegradation of solid organic wastes is presented. The pycnometer was used to observe the development of the free air space in the sample during the respirometric experiments and verified that this parameter remained favourable to the substrate oxygenation. To test this new method, respirometric experiments were conducted on four solid waste samples (a mixture of agrofood sludge and pine bark chips, a mixture of urban sludge with new and reused pine bark chips collected before and after 21 days of composting, and a mixture of household wastes).The respirometer is based on a 10 l reactor filled with the sample. It was designed to allow the control of operating conditions: temperature, moisture and aeration. During respirometric experiments, the material temperature was constant and homogeneous. However, large variations in the moisture content were measured between the beginning and the end of respirometric tests. Moreover, the development of moisture variability throughout the substrate was observed. The aeration regime was characterised as a perfectly mixed phase considering the results of retention time-distribution studies and the free air space values measured with the pycnometer. The pycnometer accuracy was confirmed by low differences between experimental and theoretical free air space values for water and spherical glass beads. Moreover, it was verified that this measurement had no influence of respirometric responses.Applying the proposed method to the four substrates demonstrated the interest in mixing the material during the respirometric tests, when the oxygen uptake rate rO2 fell to a low and stable value. Indeed, in some cases, such mixing stimulated the restart of rO2 that confirmed the presence of residual biodegradable organic matter. The major effect of mixing was to restore favourable moisture content throughout the substrate. The results revealed that a small mixing impact corresponded to either a low substrate biodegradability or a high initial moisture content. The pycnometer measurements demonstrated the influence of a low free air space. It appeared that the absence of oxygen uptake restart after mixing, in the presence of residual organic matter, could be due to a low free air space.These results demonstrated the need (i) to mix the samples to quantify all its biodegradable organic matter and (ii) to control the moisture, temperature and aeration to ensure favourable conditions of biodegradation.