Low-temperature in situ IR spectroscopy has been used to investigate the reaction of CO and NH3 over Rh/Al2O3. The presence of a strong vibrational feature at 2076 cm(-1) due to linear Rh-x(0)-CO is clearly evident at 100 K during coadsorption experiments. However, beginning at 230 K, the formation of isocyanate species was detected on both rhodium and the Al2O3 support by two distinct infrared featrures at 2170 and 2246 cm(-1), respectively. The parallel growth of these v(a)(NCO) modes is consistent with the gradual decomposition of a new band at 1970 cm(-1), which is assigned to a perturbed v(CO) mode bridge-bonded by NH surface species. This bridge-bound complex Rh-(OC<--NH) is considered to form by nucleophilic attack of Rh-NH on the carbonyl C of Rh-x(0)(CO) surface species. Upon warming, the decomposition of this complex leads to the formation of rhodium isocyanate (Rh-NCO), and the NCO ultimately migrates to the support.