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Infrared studies of NO reduction by CO over Rh SiO 2 give clear indication of the presence of Rh NCO species characterized by a band at 2170–2200 cm− 1. The surface coverage by this species is greatest under conditions where the NO partial pressure is low and the CO partial pressure is high. The NCO Groups present on Rh are very unstable and decompose immediately when the reduction of NO ceases. The in situ infrared spectra also show evidence for Si NCO groups characterized by bands at 2300 and 1460 cm− 1. These species appear to be formed by spillover of isocyanates from the rhodium microcrystallites onto the support. Once formed, the Si NCO groups are quite stable and are not readily decomposed.