The volumetric swelling procedure was applied to Turkish Soma lignite using thirteen solvents to investigate the cross-linking in its macromolecular network. The theory of solvent swelling of cross-linked polymers developed by Flory and Rehner and extended by Kovac and Peppas was used to calculate number-average molecular mass per cross-link of organic structure ((M) over bar (c)) in lignite samples. The results show that the volumetric solvent swelling of Soma lignite samples in non-polar and polar solvents roughly follows the regular solution theory.