The reduction of nitric oxide by hydrogen on rhodium was studied by utilizing surface-enhanced Raman spectroscopy (SERS) to probe the nature of adsorbed species formed underreaction conditions. As in our earlier studies, SERS-active transition-metal surfaces are prepared by electrodepositing ultrathin Rh films onto electrochemically roughened gold. These interfaces display remarkably robust SERS activity, enabling temporal sequences of surface Raman spectra to be obtained over a wide range of reactant pressures (here up to 1 atm) and at temperatures up to at least 450 degrees C. As is also reported in our recent study of the CO-NO reaction, heating Rh in pure NO yielded a surface dominated increasingly by adsorbed atomic nitrogen up to 300 degrees C, as diagnosed by a 315-cm(-1) band due to surface-nitrogen stretching. The NO-H-2 reaction was investigated at atmospheric pressure using various reactant compositions. The desorption temperature of adsorbed atomic nitrogen was observed to sharply decline as the relative amount of H-2 was increased, indicating reactive removal by adsorbed hydrogen atoms. Under conditions of either equimolar reactants or excess hydrogen, a feature at 450 cm(-1) was observed at temperatures above 100 degrees C. Substitution of H-2 by D-2 yielded no discernible downshift in the frequency of this vibration. This result, coupled with findings from a number of experiments involving transient spectral responses to changes in the gas-phase reactant composition, leads us to suggest that the 450-cm(-1) vibration arises from a Rh-NOH species formed via reaction between adsorbed NO and atomic hydrogen. The possible mechanistic importance of this species and how it may relate to reported gas-phase products and catalyst selectivity are also discussed.