The practical interest to behavior of Cs in Pb melts is caused by possibility of transmutation of long-lived isotope Cs-137 in Pb-208 carrier medium. The equilibrium potentials of Pb alloys, containing up to 10 molar % Cs, in fused cesium chloride are determined by the emf method. An activity, activity coefficients, excessive partial thermodynamic characteristics of Cs in the Pb melts are computed. A content of Cs in alloy after electrochemical experiments is determined by the method of atomic absorption spectroscopy. The activity coefficients obtained correspond to the prediction, based on regularities of changing in the alloy thermodynamic characteristics depending on dimension factor and difference of electronegativity of atoms-component of metal melts.