The effectiveness of a composting treatment of solid organic waste depends on the preparation of the initial waste and on process management. Among the characteristics of the initial waste which play an important role, the physical parameters are essential because they influence both the substrate biodegradability and the aeration conditions. Accurate and reliable measurement methods are needed to study these physical parameters. In this context, the aims of this work were, firstly to design and develop a new device to measure free air space and air permeability on heterogeneous solid waste, and secondly to calibrate and validate this apparatus in order to ensure the reliability of the method. The developed de,vice combines features of an air pycnometer, an air permeameter and a compaction device with adjustment of the wet bulk density at a given value, for the characterisation of 40-60 1 of solid samples. The results of calibration showed that the expansion of the cells under pressure and the intrinsic pressure drop of the system need to be taken into account during the measurement, For the validation of the apparatus reliability, the comparison of theoretical and experimental values of free air space and air permeability obtained with spherical beads (diameters 21 and 6 mm) showed that all experimental values were within the predicted ranges. This result was obtained irrespective of the diameter of beads and the volume, demonstrating the accuracy of the developed device. 2013 IAgrE. Published by Elsevier Ltd. All rights reserved.