The formation of an isocyanate complex in the NO + CO reaction on alumina- and silica-supported chromia was investigated by infrared spectroscopy. Isotopically-labelled gases were used to confirm that the bands at 2262 and 2242 cm^{â 1} on Cr2O3/Al2O3 are due to surface isocyanate. The effects of the experimental conditions on the intensities of the above bands were investigated in detail. Isocyanate on Cr2O3/SiO2 yielded bands at 2315 and 2210 cm^{â 1}. The band at 2210 cm^{â 1} was assigned to Cr²⁺â NCO, while the bands at 2262 and 2242 cm^{â 1} and that at 2315 cm^{â 1} were attributed to the isocyanate complex residing on alumina and silica, respectively.