Abstract The sorption of organic molecules on coals of various rank was studied using gravimetric and pycnometric techniques. The pycnometric technique was insensitive to sorption due to swelling. Hence, a comparison of sorption values obtained by these two techniques provides an accurate measure of solvent-induced swelling in porous samples. In this study the maximum degree of benzene and tetralin-induced swelling occurred in a coal of 75 wt% carbon content. Compressibility was also found to be at a maximum in this coal. These results suggest that at 75 wt% carbon content the size and/or flexibility of the chain segments between crosslinks in coal are at a maximum.