The Seebeck coefficient and electrical resistivity have been measured for the La3−xMxS4alloys with the Th3P4‐type structure forM=Sm, Eu, Yb, andx=0.1 to 0.9 in order to determine their potential as high‐temperature thermoelectric energy conversion materials. An analysis of the lattice parameters of these alloys suggests that Sm has a valence of 2.2 over the entire composition range. Furthermore, the variation of the Seebeck coefficient and electrical resistivity as a function of electron concentration is different for the Sm‐doped alloys than for the Eu‐ and Yb‐doped alloys, which are all different from that of the pure binary LaSyalloys. A maximum in the electrical power factor at 1000 °C is found to occur atx&bartil; 0.3, but when a reasonable estimate is made of the thermal conductivity the maximum in the figure‐of‐merit at 1000 °C shifts to largerxvalues (∼0.7) in all alloy systems. The long term stability of the Sm and Eu ternaries is also reported.