Thermodynamics and kinetics of the dismutation of superoxide anion (O-2(.-)) 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 (O-2(.-)) for SOD. The molar reaction enthalpies of the SOD reaction and the pyrogallol autoxidation have been measured as -160.1 and -218 kJ/mol, respectively, at 298.15 K and pH 8.0. The experimental results showed that SOD had no effect on the kinetic parameters or the mechanism for pyrogallol autoxidation. This autoxidation followed second-order reaction kinetics in the presence of limited oxygen (first order with respect to both pyrogallol and O-2), and the second-order rate constants were determined at 298.15 K and pH 8.0 to be 1.25 and 1.30 1/mol s in the absence and presence of SOD, respectively. A possible mechanism for the autoxidation of pyrogallol inhibited by SOD was also suggested.