This paper discusses moderate pressure CO2 sorption behavior of Illinois coals. The results fit the Langmuir and Dubinin-Astakhov (D-A) sorption models satisfactorily although the fit is better for D-A equation. Since factors like swelling of coal with CO2 sorption and CO2 dissolution in coal matrix contribute to uncertainties in estimating the void volume in and around the sample, an attempt was made to account for these by modifying the conventional adsorption equation. Re-fitting the experimental data using the modified equation results in improved fit for both models. The adsorption capacities of coals tested, as predicted by the equations, also reduce by 7% to 32%. The effect of volumetric uncertainty is more in lower rank coals than the higher rank ones. Furthermore, it explains the excess sorption behavior observed by others when extrapolated beyond the experimental pressure range. (C) 2008 Elsevier Ltd. All rights reserved.