Surface-enhanced Raman spectroscopy (SERS) is used as an in situ probe to study gas–solid catalytic reactions. The oxidation of CO by NO, and O2 is studied on electrochemically deposited overlayers of Rh on roughened gold substrates. A multichannel charge coupled device detector is used to obtain time-resolved SER spectra at actual reaction conditions of atmospheric pressure and temperatures up to 623 K. The gas-phase composition is simultaneously monitored with a quadrupole mass spectrometer. Correlations are made between real time measurements of adsorbed species and reaction intermediates on Rh surfaces by SERS and changes in the gas phase composition detected by mass spectrometry. For example, disappearance of adsorbed CO from the surface can be directly correlated to changes in rates of the CO oxidation reaction. Reaction intermediates of the CO and NO reaction on the Rh overlayers are observed. NO adsorption yields spectral features at 315 and 530 cm−1 attributed to the dissociation products of NO viz. adsorbed nitrogen and oxygen, respectively. There seems to be no evidence of molecularly adsorbed NO, suggesting the possibility of substantial dissociation of NO. Adsorbed reaction intermediates such as NO2, isocyanate, and cyanide species, are also observed during the CO/NO reaction at reaction temperatures. The temperature dependency of the dissociation products and reaction intermediates has also been studied. The results strongly suggest a dissociative mechanism for the CO/NO reaction at these conditions.