The electrode potential-dependent formation, stability, and nature of oxide layers formed on iridium, palladium, rhodium, and platinum in acidic aqueous solution are explored by means of in-situ surface-enhanced Raman spectroscopy (SERS). The technique utilizes recently developed procedures by which "pinhole-free" ultrathin (ca. 3-5 monolayers) transition-metal films are prepared by electrodeposition onto a SERS-active gold substrate, yielding polycrystalline Pt-group surfaces with negligible spectral interferences from the underlying gold. The oxide films formed by water electrooxidation exhibit broad yet distinctive metal-oxygen (nu(M-Ox)) lattice modes primarily in the range 500-800 cm(-1). Of particular interest is the nature and stability of oxides produced upon exposure to a chemical oxidant, nitric oxide, in acidic solution as sensed by SERS, in relation to the conventional electrochemical oxide. A distinctive oxide species identified by a sharp 570 cm(-1) nu(M-Ox) feature is formed on iridium by aqueous NO exposure, which exhibits a remarkable resistance toward reduction in comparison with oxide produced by water electrooxidation. This anomalous stability is evident not only in aqueous solution but also at elevated temperatures in dry gaseous hydrogen. The film is tentatively identified as crystalline IrO2 on the basis of its vibrational fingerprint. A spectrally distinct oxide species also displaying unexpected kinetic stability is evident on palladium exposed to NO-containing electrolyte at higher electrode potentials, although no such species are discernible on rhodium or platinum electrodes. However, the rhodium oxide is seen to survive to lower potentials in NO-containing electrolyte than in the absence of NO. These findings are ascribed to the ability of nitric oxide to act as an efficient oxygen-atom source upon dissociative chemisorption, and which, unlike water, does not require electrooxidation to form metal oxide. Another significant, although much less effective, source of oxide from aqueous solution was found to be dissolved dioxygen.