The increase in the amount of pyridine-soluble material obtained from Argonne Premium coals after acid treatment is examined. The amount of pyridine-soluble material in most of the coals increases significantly with acid treatment. In low and to some extent medium rank coals this is largely a result of the presence of ionic clusters formed by carboxylate groups. In higher rank coals we are proposing that pi-cation interaction play a major role. These ion/coal interactions are of sufficient strength to act as "reversible" cross-links, in the same way as ionic clusters behave in ionomers.