The decomposition of water and CO over clean and carbide-modified W(111) is studied by using temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and auger electron spectroscopy (AES). On both clean and modified W(111) surfaces, the activity toward the decomposition of water is found to be significantly higher than Pt group metals. For the CO experiments, both molecular and dissociative adsorption are observed on W(111) and C/W(111). Approximately 52% and 10% of the adsorbed CO dissociates to produce atomic oxygen and carbon on W(111) and C/W(111), respectively. In contrast, CO molecules undergo reversible desorption on oxygen-modified C/W(111) at temperatures as low as 242 K. Finally, coadsorption experiments of water and CO on C/W(111) show that the presence of surface hydroxyls hinders the adsorption of CO, and that only trace amount of gas-phase CO2 is detected.