Swelling of coal, especially in the presence of CO2, may reduce the permeability of coal seams thus affecting the viability of enhanced coalbed methane production or CO2 sequestration operations. In this paper we report laboratory measurements of swelling in four Australian bituminous and sub-bituminous coals in CO2, methane and various mixtures of the two gases. Measurements were made on unconstrained monolithic samples at 55 degrees C and at pressures up to about 15 MPa. Volumetric swelling at 15 MPa ranged from about 1.9% to 5.5% in CO2 and 1.0% to 2.5% in CH4 depending on the rank of coal and the proportion of CO2 in the gas mixture. Swelling measurements made at a constant pressure of 15 MPa but with varying gas composition showed that even CO2 with a high affinity for coal could be completely displaced by helium, which does not adsorb, causing the coal sample to shrink to its original dimensions. Similarly, CH4 was displaced by injecting CO2 into the system which then caused the coal to swell to the same level as if it had been exposed to pure CO2. The results of these experiments show that there is no enhanced swelling in mixed gases above that would be observed in the pure CO2 at the same total pressure. As well, the swelling behaviour of coal in mixed gases was determined solely by the partial pressure of the sorbing gas so that weakly or non-sorbing gases are effective at displacing strongly adsorbed gases. (C) 2012 Elsevier B.V. All rights reserved.