The pyrrhotite solvus that represents the compositions of pyrrhotite coexisting in equilibrium with pyrite was determined in the temperature range 325 degrees C. to 743 degrees C. by experiments conducted in sealed, evacuated, silica glass capsules and at pressures equal to that of the vapor in equilibrium with the condensed phases. Experiments conducted in sealed, collapsible Au tubes demonstrate that confining pressures of 2000 bars do not measurably affect the position of the solvus below 670 degrees C. The compositions of synthetic hexagonal pyrrhotite were measured within + or -0.13 atomic % Fe with the aid of an X -ray determinative curve that relates d (102) to composition. X-ray powder data and a general description are given for an unidentified lamellar iron sulfide phase occurring in rapidly quenched Fe-deficient pyrrhotite. Temperatures of crystallization of 10 natural pyrrhotite-pyrite assemblages are estimated using the pyrrhotite solvus. The temperature of crystallization of sphalerite coexisting with pyrrhotite and pyrite in 4 of these samples was also measured; the estimates obtained from the 2 methods, with very few exceptions, agree well within the experimental error.