The Seebeck coefficient of a cell with lead electrodes and electrolyte of molten lead chloride and alkali chloride is investigated experimentally and theoretically for mole fractions of PbCl2 between l and 0.3 in alkali chlorides. An equation is derived from irreversible thermodynamics linking the transported entropy of lead ion to the chemical potential of the polarized lead at the surface. The path across the cell is taken to be adiabatic in this derivation. Experimental results indicate that the transported entropy of lead ions depends on the radius of the alkali ion.