Mawsonite and stannoidite characteristically occur with bornite and chalcopyrite in xenothermal ore deposits in Japan. The two minerals are not found in hypothermal ore deposits in which stannite occurs. Data on the thermal stability and phase diagrams of the system Cu-Fe-Sn-S including mawsonite and stannoidite are limited. Therefore, synthetic and experimental studies, especially of mawsonite and stannoidite, were carried out to obtain information on the genesis of the two minerals in nature. Phase diagrams of a part of the system Cu-Fe-Sn-S have been constructed at 700 degrees , 500 degrees , and 300 degrees C from the results of dry, flux, and hydrothermal syntheses. Differential thermal analyses and heating experiments were also carried out. The results indicate that mawsonite is stable at temperatures below 390 degrees C and that it decomposes to stannoidite, bornite, and chalcopyrite above this temperature. Mawsonite coexists with stannoidite, bornite, and chalcopyrite in the synthetic products as well as in the natural occurrence. Although mawsonite does not contain appreciable amounts of zinc, natural stannoidite generally contains some zinc. According to the results of heating experiments, natural stannoidite containing 3 weight percent of zinc decomposes to bornite, stannite, and chalcopyrite at about 500 degrees C, while zinc-free synthetic stannoidite is stable up to 800 degrees C. The temperature range of stability of zincian stannoidite may vary greatly with the zinc content. The sulfur fugacity determination by the electrum tarnish method indicates that the univariant assemblage of mawsonite, stannoidite, bornite, and chalcopyrite requires a higher sulfur fugacity than that of stannoidite, stannite, bornite, and chalcopyrite. It is also concluded from the results of experiments and observation of natural ores which carry mawsonite and stannoidite that mawsonite could have been formed by replacing stannoidite at temperatures lower than 390 degrees C and at a higher sulfur fugacity than the assemblage of stannoidite, bornite, and chalcopyrite.