The kinetic activities of the heterogeneous decomposition of hydrogen peroxide by metal–iron spinel oxides, M_xFe_3–xO₄(M = Cu or Ni), have been investigated with a view to defining the effects of composition and microstructure on their catalytic activity. The entire composition range, 0 < x < 3, was prepared and characterized. Although the series of nickel catalysts was found to possess a lower aggregate diameter than the copper series, the decomposition activity of the former was surprisingly much less than the latter. This poor performance of the nickel series is explained in terms not of a consideration of microstructural defects but rather of the restricted redox couple represented by Mⁿ/M^n–1 in the electronic composition of the catalysts and, possibly, the absence of a more active ion (Mⁿ) at high compositions on the octahedral lattice sites which may initiate the cyclic electron-transfer process on the catalyst surface, as proposed by Abel and Cota et al. The value of x corresponding to maximum activity was experimentally evaluated for each catalyst series by determining the highest specific rate constant having minimum values of the Arrhenius pre-exponential factor, rather than minimum values of the activation energy.