Attempts to describe high-pressure carbon dioxide (CO2) adsorption isotherm data using conventional adsorption equations to model the coal behavior have been only partially successful. Because swelling of the coal organic matrix in the presence of adsorbing gases is a well-known phenomenon and because traditional isotherm models assume a rigid structure, an adsorption isotherm equation was derived to account for the volume effects which may occur when an adsorbate alters the structure of an adsorbent. The equation, which accounts for volume change in general, was applied to the particular example Of CO2 adsorption on coal. In some cases, significantly better fits were obtained when the adsorption data were fit to the swelling-modified model. The modified model partitions the experimentally determined adsorption into a surface adsorption term, which is important at lower pressure, and a rectilinear term, related to volume effects, which is important at higher pressures. This is particularly significant to the problem of CO2 sequestration in coal seams where high pressures of CO2 will be used.