A series of Mn-Cr-O spinel catalysts of different stoichiometry was synthesized using a sot-gel method and characterized using X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, N-2 and H2O adsorption, and H-2-temperature programmed reduction. The introduction of extra Mn to stoichiometric MnCr2O4 led to Mn3+ substitution in the spinel lattice of Mn1.5Cr1.5O4 whereas excess Cr in Mn0.5Cr2.5O4 formed Cr2O3. Ethylene steam reforming at atmospheric total pressure and 873 K over the Mn-Cr-O spinel catalysts was nearly first order in ethylene, negative order in water and zero order in excess dihydrogen. The reaction kinetics were consistent with a Mars-van Krevelen type mechanism. Whereas Cr2O3 was nearly inactive for ethylene steam reforming, Mn3O4 was substantially more active than the spinel catalysts, but Mn3O4 deactivated significantly upon in situ reduction to MnO. The spinel catalysts were therefore more active and structurally stable for ethylene steam reforming than either of the pure metal oxides. (C) 2019 Elsevier Inc. All rights reserved.