From a comparison of high and low pressure sorption behaviour of 28 bituminous and subbituminous coals for carbon dioxide, the sorption capacity calculated at high pressure is always substantially greater than that estimated from low pressure sorption measurements. The difference between maximum sorption capacity from high pressure measurements and that from low pressure measurements increases with decreasing rank. This difference can be quantitatively explained by swelling of the coal at high pressure that does not occur during low pressure measurements. When expressed as volume %, the maximum sorption capacity calculated from high pressure measurements was found to equal the sum of the maximum sorption capacity calculated from low pressure measurements and the volumetric swelling the coal undergoes on exposure to high pressure. This relationship implies that the volume occupied by the coal molecules is constant when it swells: the greater apparent coal volume that occurs on swelling in gases is entirely taken up completely by increased pore volume. Moreover, this relationship provides a natural explanation for the finding that when a coal that is swollen with gas is compressed, the coal releases it. If so, low pressure sorption measurements may provide a more direct estimation of coal sorption capacity in constrained coal seams, provided a robust method of predicting maximum sorption capacity from low pressure sorption behaviour can be established. (C) 2011 Elsevier B.V. All rights reserved.