The selectivity in cation exchange of Loy Yang brown coal treated at 50-250 degreesC was studied in an aqueous solution containing two kinds of metallic ions to clarify the effects of the heat treatment temperatures of coal, pH of the exchange solution, and addition of acetone to the exchange solution. Four divalent metallic ions of Co2+, Mg2+, Ni2+, Ca2+, mainly the former two, were used as exchange cations. The amount of Co2+ adsorption (the extent of cation exchange for Co2+, Co-ECE) from the Co-Mg mixed solution at pH 6 for the coal treated at 50 degreesC was 1.15 mmol g-coal(-1), almost the same value as that from the single Co2+ ionic solution, whereas the Mg-ECE from the Co-Mg mixed solution decreased to 1/6 of that from the single Mg2+ ionic solution which was 0.8 mmol g(-1), indicating that the ability of cation exchange of Co2+ is much higher than that of Mg2+, the selectivity of Co2+ (the ratio of the Co-ECE to the sum of the Co-ECE and the Mg-ECE) about 90%. With decreasing pH of the solution, however, the selectivity of Co2+ decreased. The selectivity of Co2+, the low pH region between 2 and 5.5 decreased further for the heat-treated coal samples at higher temperatures. On the other hand, in the higher pH region, the selectivity of Co2+ was recovered to larger than 70% independent of heat treatment temperatures. By addition of acetone to the exchange solution, the selectivity of Co2+ was also recovered up to about 50% even in the low pH region. These changes of the selectivity in cation exchange are discussed based on the structural changes of brown coal. (C) 2001 Elsevier Science Ltd. All rights reserved.