As a result of studying the kinetics, products and volume yield of free radicals, a scheme is proposed for the mechanism of breakdown of tert-butylhydroperoxide, catalysed by Co(II) and Co(III) acetylacetonates. The kinetic dependences correspond to a mechanism with the intermediate formation of a complex decomposing into products. The equilibrium constant K = 6.9 l/mole is not dependent on temperature and is the same for the two catalysts. The rate constants of decomposition of the catalyst-hydroperoxide complex at 343-363 K are respectively k(Co(II)) = 1.05 x 10(14) (exp - 92 kJ . mole(-1)/RT) sec(-1) k(Co(III)) = 1.92 x 10(12) (exp - 92 kJ . mole(-1)/RT) sec(-1). The equality of constants K and of the activation energies of decomposition of intermediate complexes for Co(II) and Co(III) with ROOH indicates that, irrespective of the initial form of the catalyst in the two cases breakdown is carried out by the same intermediate catalytic form. The radical yield is 50%.