The potentiometric and polarization measurements are carried out in the system of Mo-sodium boroaluminate melt with additives of molybdenum oxide. A dependence of equilibrium electrode potential of Mo electrode and magnitude of ultimate cathode current on concentration of Mo oxide in the melt is shown. A mechanism of the electrode processes, occurring at the Mo electrode in the oxide melt is suggested. It is inferred that the relatively high polarizability of Mo electrode restricts a possibility for it using as the accessory electrode in high-temperature research of electrochemical processes.