Poly(5-acrylamidosalicylic acid) acts as a monobasic acid with a dissociation constant of 3.13 × 10 −4 moles l −1 derived from the direct titration of monomeric 5-acrylamidosalicylic acid. The polymer exhibits very low solubility in acid media but is appreciably soluble above pH 6, when it is almost wholly dissociated. Studies with acetate, chloride, nitrate and sulphate ions show that the exchange characteristics of the polymer are little affected by the nature of the counter anion present in the case of strongly bound cations, but exchange levels are markedly depressed by the presence of sulphate in the case of weakly bound cations. Possible reasons for this change in behaviour are discussed.