It has been shown that the isolated -OH groups present on a Rh/Al2O3 catalyst form a weak I-I-bonded complex by exposure to gas-phase NH3 at 298 K. This reversible complex suppresses the tendency for isolated -OH groups to participate in the oxidation of metallic Rh, forming Rh-1(CO)(2)/Al2O3 species in the presence of a CO atmosphere. There is no IR spectroscopic evidence for the formation of NH4+ during surface complexation. However, evidence for NH2 formation at Lewis base sites on Al2O3 is found.