Samples of the spinel series, Mn_xFe_{3 –x}O₄(O ⩽x⩽ 3), have been synthesized by the hydroxide coprecipitation technique and characterised by chemical analysis; XRD and SEM techniques, and measurements of their BET specific surface areas, saturation magnetisations and electrical resistivities. The specimens with values of x⩽ 1.5 are single-phase cubic spinels and those with x > 1.5 are two-phase cubic and tetragonal spinels. The relative catalytic activities of the specimens towards the decomposition of H₂O₂ in both alkaline and neutral media, measured on the basis of the energies of activation, a microstructure-independent parameter, and the rate constants normalised to unit BET surface area, a microstructure normalised parameter, follow the order, (given as values of x) 1.45 > 2.5 ≈ 2.0 > 1.0 > 1.2 > 0.8 > 0.5. All the Mn_xFe_{3 –x}O₄ prepared specimens are potential substitutes for CoFe₂O₄, the high-performance catalyst for the decomposition of H₂O₂; the specimen with x= 1.45 is the most powerful and is significantly more active than CoFe₂O₄. The activity appears to depend on the availability and efficacy of redox conversion of redox conversion of Mn^II centres in the sublattices of the Mn_xFe_{3 –x}O₄ catalysts.

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