Thermodynamics and kinetics of the dismutation of superoxide anion (O2) catalyzed by superoxide dismutase (SOD) in batch reactors has been studied by on-line reaction calorimetry. The decomposition of hydrogen peroxide catalyzed by catalase is utilized as a source of oxygen and the autoxidation of pyrogallol as a source of the substrate (O2) for SOD. The molar reaction enthalpies of the SOD reaction and the pyrogallol autoxidation have been measured as! 160.1 and !218kJ/mol, respectively, at 298.15K and pH 8.0. The experimental results showed that SOD had no e!ect on the kinetic parameters or the mechanism for pyrogallol autoxidation. This autoxidation followed second-order reaction kinetics in the presence of limited oxygen ("rst order with respect to both pyrogallol and (O2), and the second-order rate constants were determined at 298.15K and pH 8.0 to be 1.25 and 1.30l/mol s in the absence and presence of SOD, respectively. A possible mechanism for the autoxidation of pyrogallol inhibited by SOD was also suggested. (2000) Elsevier Science Ltd. All rights reserved.