The development of effective breeding blankets is crucial to the success of future fusion reactors. One of the main liquid breeding candidates is molten eutectic Pb-Li. In this environment, it is essential to control lithium concentration and dosage to keep the adequate composition in the melt. In this frame, on-line and on-board lithium monitoring tools, which can operate under extreme conditions, are required. Electrochemical sensors based on lithium conducting solid electrolytes (Li6La3Ta1.5Y0.5O12 and Li6BaLa2Ta2O12) were designed and tested. These potentiometric sensors were used to measure the Open Circuit Potential (OCP) in molten lead-lithium alloys with different lithium concentrations. Measurements were performed at 400 °C, 500 °C, and 600 °C. Experimental OCP values were in good agreement with those calculated with the Nernst equation. OCP values showed a linear relationship versus the logarithm of the lithium activities, fulfilling the Nernst equation.