The adsorption of potassium and CO₂ and their interaction on a Au(111) surface have been studied using high-resolution electron energy loss spectroscopy, thermal desorption spectroscopy, work function measurements, and Auger electron spectroscopy. Potassium adsorbs in cationic form on Au(111) at low coverage, depolarizing to a neutral metallic state at high coverage. CO₂ adsorbs only weakly on clean Au(111), desorbing with a T_p ∼ 124 K. Its binding energy is greatly enhanced by the presence of potassium and led to the formation of a CO₂⁻ anion radical characterized by losses at 957, 1360, and 1620 cm⁻¹. An enhancement of a weakly held, unperturbed CO₂ also occurred on a K-dosed surface, which was attributed to the formation of an unstable cluster compound (CO₂)_n·CO₂⁻. The surface concentration and the reactivity of the CO₂⁻ radical sensitively depend on the K coverage and on the state of potassium. At low K coverages, CO₂⁻ dissociated into CO_(a) and O⁻_(a). At higher coverages, it disproportionated into stable CO₃⁻_(a) and CO_(a)⁻; they decomposed and released only above 500 K. CO formed on a K-dosed surface gave vibration losses at 1940−1700 cm⁻¹ and was also stabilized by potassium.