Surface-enhanced Raman spectroscopy (SERS) combined with X-ray photoelectron spectroscopy (XPS) has been utilized to study the oxidation of ruthenium at ambient pressure (1 atm) and elevated temperatures (25-300 degrees C). The SERS probe provides in-situ vibrational information regarding surface oxide bonding. While the XPS probe necessarily involves ex-situ measurements (i.e., transfer to and from ultrahigh vacuum), it provides valuable complementary information on the metal and oxygen electronic states. Ruthenium surfaces were prepared by electrodepositing ultrathin films (about three monolayers) onto electrochemically roughened (i.e., SERS-active) gold substrates. Insight into the in-situ oxidation process was obtained by probing the changes of surface speciation by SERS upon heating Ru in flowing O-2. A pair of SERS bands at 470 and 670 cm(-1) appear in the spectrum acquired for a freshly electrodeposited film, which are assigned to different stretching modes of hydrated RuO2 formed during sample transfer to the gas-phase reactor. However, a fully reduced Ru surface (i.e., devoid of oxide features) could be formed by adsorbing a protective CO adlayer in an electrochemical cell followed by heating to 200 degrees C in vacuum so to thermally desorb the CO. While the initially oxidized (i.e., RuO2) surface was stable to further oxidation upon heating in O-2, adsorbed atomic oxygen was detected at 200 degrees C from the appearance of a SERS band at 600 cm(-1) and a XPS O(1s) peak at 531.7 eV. In contrast, the higher oxides RuO4 and possibly RuO3 were produced only upon thermal oxidation of the fully reduced Ru surface. Evidence for RuO3 formation includes the appearance of a 800 cm(-1) SERS band at 200 degrees C which correlates with the advent of a Ru(3d(5/2)) peak at 282.6 eV. The surface was further oxidized to RuO4 at 250 degrees C, as deduced from the formation of a 875 cm(-1) band and a Ru(3d(5/2)) peak at 283.3 eV. While RuO3 and RuO4 were exclusively formed at temperatures higher than 250 degrees C, RuO2 was produced upon cooling to room temperature, possibly via the decomposition of RuO4.