A hydrothermal recrystallization method was employed to determine the phase relations in the Fe-S system involving pyrrhotite between 350 degrees and 115 degrees C. The superstructures of pyrrhotite, as designated by Nakazawa and Morimoto (1970, 1971), are considered to be thermodynamic phases. The solvus between pyrite and hexagonal (1C) 1 pyrrhotite continues below 325 degrees C along the slope established by Toulmin and Barton (1964) to 308 degrees C, where a discontinuity marks the upper stability of the MC superstructure. The solvus then continues along a similar slope to 262 degrees C where, with the onset of the NA superstructure, there is a discontinuity and the curve reverses slope sharply. At 254 degrees C, monoclinic (4C) pyrrhotite becomes stable in the peritectoid reaction hexagonal (NA) pyrrhotite (47.30 at. % Fe) + pyrite = monoclinic (4C) pyrrhotite (47.25 at. % Fe). Monoclinic pyrrhotite exists over a range of compositions, the maximum being 46.4 to 46.9 at. percent Fe at 115 degrees C. Hexagonal pyrrhotite is separated from monoclinic pyrrhotite by a narrow miscibility gap down to the upper stability limit of the NC superstructure at 209 degrees C. Below 209 degrees C, the miscibility gap widens, with hexagonal (NC) pyrrhotite of 47.3 at. percent Fe coexisting with monoclinic pyrrhotite of 46.9 at. percent Fe. At the sulfur-rich limit of the stability field, monoclinic pyrrhotite coexists with pyrite. The phases greigite, smythite, and marcasite were not encountered.Our results are in reasonable agreement with those obtained previously by hydrothermal synthesis methods, but they are in disagreement with some results obtained in "dry" experiments which have produced metastable phase assemblages. The phase diagram is compatible with natural pyrrhotite occurrences and in particular with those from geothermal boreholes at Broadlands, New Zealand, from which samples were recovered at known quench temperatures. On the other hand, our studies cast considerable doubt on the validity of the "pyrrhotite geothermometer" as applied to ores.