Series of chromium spinels MCr2O4 (M = Mg, Zn, Fe, Co, Ni, Cu, Mn) were tested in the selective catalytic reduction (SCR) of NO with NH3. Catalytic activity and selectivity to N2O depend on the nature of the divalent metal M. The studies by TPR. XPS and chemical analysis lead to the conclusion that the surface layer of all chromium spinels decomposes to simple oxide species containing M2+/M3+ and Cr3+/Cr6+ redox systems. An increase in the oxidation potential of the redox system is accompanied by a decrease in the selectivity to N2O in agreement with the mechanism of the reaction proposed by the Niiyma group. (C) 2000 Elsevier Science B.V. All nights reserved.