The utility of surface-enhanced Raman spectroscopy (SERS) as an in situ adsorbate probe of heterogeneous catalytic systems at high gas pressures there up to 1 atm) in conjunction with mass spectrometry (MS) is demonstrated for reactions on transition-metal catalysts, at temperatures up to 500 degrees C. This simultaneous SERS-MS approach enables the relationships between the formation of specific adsorbed species (as sensed by SERS) and reaction products las detected by MS) to be explored on a common (approximate to 1 s) time scale. The low-frequency (<1000 cm(-1)) spectral region, where metal-adsorbate vibrations are located, is especially informative. The environmentally important reactions of NO reduction by CO or H-2 and the oxidation of methanol on transition metals (Rh, Pd, Pt) are considered here. The possible mechanistic significance of surface oxide formation observed during such ambient-pressure catalytic reactions is also discussed.