Chelex 100 chelating resin, as supplied (mixed sodium/hydrogen form 43% /57%), was found to be readily hydrolyzable through hydronium ion exchange from water. Contact with deionized water in solution volume to exchanger weight ratios of 10-4000 brought sodium ions into solution with concentrations of 0.05-1.5 mM and increased the pH from 6.1 to 8.7-10.0. In addition to sodium and hydroxide, the exchanger was found to release chloride and carbonate ions. The two acetate groups vary substantially in their selectivities to hydronium ions, the corrected selectivity coefficients being 1.3 x 10(9) and 1.4 x 10(3), respectively.