The hydrolysis behavior of a number of polymers of bithionol [2,2′-thiobis(4,6-dichlorphenol)] was investigated. The hydrolysis of polyesters of bithionol with aliphatic or aromatic dicarboxylic acids or phosphorus acids as well as polycarbonates and alternating copolycarbonates and polyurethanes or co(po1y-carbonates/polyurethanes) depends primarily on the water solubility of the polymer or at least on its degree of swelling. In the most favorable case of the alternating bithionol/PEG 4000 co-polycarbonate the hydrolysis rate of the polymer at 37°C was 1.62 L/mol/min at pH 10, 0.63 L/mol/min at pH 4 and 0.17 L/mol/min at pH 7.4. Copolycarbonates and copolyurethanes with PEG 4000 in the polymer chain have slightly lower rates of hydrolysis. When the solubility of the polymer is low and the crystallinity