Surface-enhanced Raman spectroscopy (SERS) has been utilized as an in-situ probe of adsorbed species and surface reaction intermediates during the reduction of nitric oxide by either carbon monoxide or hydrogen over polycrystalline Rh films. SERS-active Rh surfaces were prepared by electrodeposition of ultrathin films on electrochemically roughened gold and display remarkably robust SERS activity over a wide range of temperatures in conjunction with SERS, enabled (up to 400 degrees C) and pressures (here up to 1 atm). Mass spectrometry, employed in conjunction with SERS, enabled simultaneous real-time measurement of reaction kinetics for the CO-NO reaction. A charge-coupled device simultaneous real-time measurement of reaction kinetics for the detector (CCD) allowed Raman spectra to be recorded on a time-scale (less than or equal to 10 s) commensurate with reactions occurring on the surface. Several central differences exist between these two reduction processes, most notably the mechanism for NO dissociation. While NO decomposition proceeds through a direct pathway (NO(ads)+S double right arrow N-(ads)+O-(ads)) and is largely unaffected by the relative amount of gaseous CO, a hydrogen-assisted pathway appears to be prevalent during NO reduction hydrogen-rich environments. evidence suggests that this process proceeds via a Rh-NOH intermediate (450 cm(-1)). Adsorbed atomic nitrogen (315 cm(-1)) reacted only to form N-2 during reduction with CO, adjudged by its removal temperature (325 degrees C) and the absence of N2O formation. In contrast, hydrogen facilitated the reactive removal of this moiety at lower temperatures, most likely via NH3 formation. While extensive surface oxidation was detected during reaction with varying NO/CO ratios, Rh2O3 formation was inhibited under hydrogen-rich mixtures. These differences in surface speciation and their probable roles in the determination cb product selectivity are discussed.