After observing that pH dropped dramatically when chloride-form strong-base anion resins were contacted with neutral and alkaline solutions of bicarbonate and arsenate anions, three representative strong-base resins were tested for their ability to produce pH changes. It was discovered that all three chloride-form resins could convert HCO 3− − to CO 3 2−, and H 2AsO 4 − to HAsO 4 2− with the expulsion of protons, a decrease in pH, and an increase in the ionic concentration of the aqueous phase. For bicarbonate which can give up and take up protons, the following reaction was observed: 2 R 4N +Cl − + HCO − 3 ⇔ (R 4N +) 2CO 3 + Cl − + H +Cl − Depending on the local concentration of excess bicarbonate, the HCl produced either remained as HCl or reacted to form carbonic acid in a reaction which tended to buffer the system. The resin characteristics related to its preference for divalent ions and its ability to produce higher-valent anions by proton expulsion were (a) appropriate functional group charge spacing, (b) ability to reorient functional groups to satisfy polyvalent ions, and (c) presence of hydrogen-bonding hydrophilic groups such as carbonyl and hydroxyl. IkA-458, the resin that was clearly superior in its ability to produce higher-valent ions, exhibits the highest divalent selectivity, is hydrophilic, and carries its quaternary amine functional groups at the end of a long flexible pendant arm containing an amide bond with hydrogen-bonding potential. Implications of the experimental findings as applied to arsenate removal from water are discussed.