In order to clarify the reaction mechanisms for oxygen reduction related to superoxide and peroxide ions in molten carbonates, we have attempted to obtain in situ Raman spectra on a gold electrode by a surface-enhanced Raman scattering technique at the three-phase boundary under potentiostatic conditions. In situ experiments were carried out with a gold electrode in molten (62 + 38) mol% (Li + K) CO3 eutectic at 923 K and in molten Li2CO3 at 1123 K. Raman lines at 1047, 1080, and 800 cm(-1) appeared for the (62 + 38) mol% (Li + K) CO3, and at 735 cm(-1) for the Li2CO3 single melt. These Raman lines are assigned to the stretching vibration of superoxide ions (1047 and 1080 cm(-1)), and peroxide ions (800 and 735 cm(-1)). These oxide species may be associated with the oxygen reduction at the three-phase boundary interface. The results suggest the oxygen reduction in a Li2CO3 melt involves peroxide ions, while the reduction in a (62 + 38) mol% (Li + K) CO3 melt involves both peroxide and superoxide ions. (C) 2004 The Electrochemical Society.