Surface-enhanced Raman spectroscopy (SERS), combined with simultaneous mass spectrometric measurements, has been utilized to probe the reactive nature of surface species present during the reduction of NO by CO on Pt and Pd. As in our earlier studies, the SERS-active transition-metal surfaces are prepared by electro-depositing ultrathin films onto electrochemically roughened gold. These surfaces display remarkably robust SERS activity, enabling intense Raman spectra to be obtained over a range of reactant pressures (here up to 1 atm) and at temperatures up to at least 400 degrees C. During nitric oxide adsorption at 1 atm on Pt, both terminal (240 and 470 cm(-1)) and bridged (325 cm(-1)) states of molecular NO were detected at lower temperatures (25-200 degrees C), with some dissociation occurring at higher (ca 250 degrees C) temperatures as evidenced by the presence of atomic nitrogen (295 cm(-1)). Similarly, a bridged NO species (310 cm(-1)) was observed on Pd under similar conditions, with dissociation detected in the form of atomic nitrogen (285 cm(-1)) and surface oxide (450 and 665 cm(-1)). Go-dosing of reactants on platinum produced a surface dominated by NO and CO (470 and 2080 cm(-1)), whereas the former was adsorbed preferentially on Pd. Simultaneous SERS/MS measurements were performed during reaction of an equimolar reactant mixture at 1 atm of total pressure over both metals. Both CO2 and N2O were formed during reaction on Pt, with onset of detectable product formation correlating with depletion of adsorbed CO and NO, respectively. In contrast, CO2 was the only product detected over Pd, with the depletion of surface oxygen suggesting that NO dissociation may be rate limiting at higher temperatures (ca 300 degrees C). The extent of dissociation on these surfaces is compared and contrasted, with particular emphasis placed on its role in determining reaction selectivity. Furthermore, the overall behavior of these catalysts is compared with our former observations regarding this reduction process on rhodium.