The n-type alloys between PbSe and PbS are studied. The effect of alloy composition on transport properties is evaluated and the results are interpreted with theories based on random atomic site substitution. The alloying in PbSe_(1−x)S_x brings thermal conductivity reduction, carrier mobility reduction as well as change of effective mass. When all these factors are evaluated, both experimentally and theoretically, the optimized thermoelectric performance is found to change gradually with alloy composition. High zT can be found in all PbSe_(1−x)S_x alloys. The possibility of achieving significant improvement of zT through alloying is also discussed
