Oxygen activity and its control is an important parameter in the use of molten LBE or lead as a coolant in ADS reactors. Oxygen concentration in the molten alloy should be high enough to form and maintain a stable passivation layer on the surface of the structural material. This layer can minimize the corrosion problems on the structural material. If the oxygen activity drops below a certain level, the passivation layer may decompose and the base metal of the structural material might be exposed to the molten alloy. Nickel, chromium and iron, constitutive elements of stainless steels, show a high solubility in LBE. If the structural material is submerged in molten LBE without a passive oxide layer, it will be dissolved by the action of the molten alloy. Potentiometric sensors for measuring oxygen activity in molten 44.5% lead - 55.5% bismuth alloy have been developed at "Institut Quimic de Sarria" Electrochemistry Lab.