The kinetics of decomposition of H2O2 catalyzed by Fe3+ in aqueous solution in the presence of HNO3 has been followed by permanganate titration and studied by the initial-rate method. In the experimental range [H2O2](0)/[Fe3+] = 41-2028 used in this study, the initial rate follows the law nu(0) = {k([Fe3+](3/2) [H2O2](0)(1/2))/ ([H+](3/2)}/{(1 + k'([Fe3+]/([H2O2](0)[H+])))(1/2)}. Both rate constants k and k' decrease with increasing ionic strength, and the corresponding apparent activation energies are 146 +/- 10 and 88 +/- 21 kJ mol(-1). The experimental rate law strongly suggests a radical-chain mechanism, with Fe2+, HO., HO2., and O-2(.-) as propagating intermediates. At high values of the [H2O2](0)/[Fe3+] ratio two different reactions compete as chain-termination steps (dismutation of HO2. radicals and reduction of HO2., by Fe2+), whereas at lower values of that ratio a third chain-termination step (reduction of HO. by Fe2+) also contributes. Thus, the kinetics of this reaction can be considered as an indirect proof of the participation of hydroxyl radicals in the mechanism.