Weak-, moderate-, and strong-base resins were evaluated for their sorption capacities of lactic acid from solutions with different pHs. Composite isotherms and breakthrough curves indicated that the sorption capacities of weak- and moderate-base resins decreased markedly as the pH of the feed exceeded the pK(a) of lactic acid. The decrease in capacity was mainly the result of the decrease in concentration of undissociated lactic acid, the species preferentially adsorbed by the resin, as the pH of the feed increases. The sorption capacity of the strong-base resin remained constant in the same pH range (pH 2-6).