The objective of this work is to study the Li2CO3Na2CO3-K2CO3 (29.5–31.1–39.4 mol%) molten salt and the behaviour of several metals (Au, Pt, C, Ni and W) by electrochemical measurements in the salt under three atmospheres (Ar, CO2 and O2/N2). Thermodynamic calculations show that the molten salt anodic and cathodic limits correspond to the carbonate ions oxidation and reduction to dioxygen and carbon respectively. An internal reference electrode has been defined based on the redox system Na+/Au2Na, which is independent of the lithium oxide activity. The widths of the electroactivity domain have been measured on gold working electrode by cyclic voltammetry: 2.21 V under Ar and O2/N2 and 2.41 V under CO2, which are larger than the values obtained by thermodynamic calculations (1.280 V and 1.025 V respectively). Gas chromatographic (GC) analysis during electrolysis at potentials ranging in the anodic limit allowed to conclude that the electrochemical domain of the carbonate molten salt is limited anodically by the oxidation of carbonate ions to dioxygen, possibly peroxide ions, and carbon dioxide. The characterisation by XRD of a platinum electrode after electrolysis at the cathodic limit has shown that the electroactivity domain of the molten salt is defined by the carbonate ions reduction into carbon, indicating that CO32−/C is an irreversible system. A carbonate ions decomposition rate of 1 × 10-3 mmol.h-1.cm−2 has been determined by GC and thermogravimetric analysis (TGA) under inert gas. A stabilization of the CO2 pressure is observed after the melting of the salt.