The distribution of acid strength of carboxyl groups in raw and heat-treated Australian Loy Yang brown coals was estimated by the sodium titration method. The acidity of raw coal varies widely. The total amount of carboxyl groups for heat-treated coals is reduced by thermal decomposition, but the distribution of the acidity barely changes. Also, the interaction between surface functional groups of brown coal and metal cations was examined by cation exchange experiments. For all cations, the extent of cation exchange (ECE) of the raw coal increased with pH, which suggests that the ECE for raw coal is controlled mainly by the acid dissociation equilibrium of carboxyl groups. For magnesium and cobalt ions, the ECE＇s of heat treated coals were smaller than those estimated from thermal decomposition of the carboxyl groups. In contast, the ECE＇s in water-organic mixed solution were larger than those in aqueous solution. ECE＇s increased linearly with swelling of coal caused by addition of organic compounds. The gradient of this increase was larger for cobalt than for magnesium and also the slope for samples treated at higher temperatures or measured at higher pH levels. These results are discussed in terms of the change of the pore size distribution by the heat treatment of the coal as well as swelling of coal by addition of organic compounds which causes a change in the cation accessibility to the carboxyl groups. From these results, the variation of the macromolecular network structure is considered to be an important factor for the cation exchange properties of brown coal in addition to chemical factors, such as acid dissociation equilibrium.