The binding of Ca2+, Pb2+, and Cu2+ ions to galactaric and 2,5-furandicarboxylic acids and of Pb2+ and Cu2+ ions to D-galacturonic acid, its α-methylglycoside, and (methyl-4-deoxy-α-D-galactopyranosid)uronic acid was evaluated in terms of the activity coefficients of the counter-ions and the degrees of their association with the carboxylate groups of the acids. The activity of Ca2+ ions was determined by the metallochromic indicator method with tetramethylmurexide, that of Pb2+ and Cu2+ ions, with ion specific electrodes. Galactaric acid bids 89.5% Ca2+ ions, practically all (99.9%) Pb2+ ions and nearly all (98.6% Cu2+ ions, mostly with the formation of insoluble salts. As little as 7.6% Ca2+, 34.0% Pb2+, and 23.9% Cu2+ were bound to 2,5-furandicarboxylic acid in the form of soluble complexes, and slightly more, 4.1% Ca2+, 43.9% Pb2+, and 38.0% Cu2+, to D-galacturonic acid. The difference in the degrees of binding to galactaric acid and 2,5-furandicarboxylic acid is due to the different flexibility of their molecular skeletons. The activities of Pb2+ and Cu2+ ions in solutions of salts of D-galacturonic acid and its derivatives indicate that the OH group at C(4) of D-galacturonic acid takes part in the complexation. For the Pb2+ complex, the OH group at C(1) of the unsubstituted acid in the β-anomeric form also seems to participate in the complexation; this is not the case with the similar Cu complex.