The cation exchange properties of Australian brown coals were investigated aiming at clarifying the influence of surface treatments. Dry heat treated (100-300 degrees C, 2 h) and hydrothermally treated(130-300 degrees C, 1 h) samples were used. Ion exchange experiments were performed by soaking coals in the solutions of calcium, magnesium, nickel, zinc and iron salts at 500-10 000 ppm in metal element, varying the initial pH by the addition of ammonia or hydrochloric acid. Extent of cation exchange (ECE) increased with increasing pH of solution. The decreases in ECEs for the dry heat treated and hydrothermally treated coals with increasing temperature were different from the decreases in the amount of carboxyl groups measured by titration method or infrared absorption spectroscopy. Further, the ECEs decreased by heat treatments were found to recover by the addition of ethanol to a certain extent. It was concluded that the main factor determining ECE on brown coals is a cation＇s accessibility to carboxyl groups which varies significantly with the pH of solution.