The reaction pyrite <--> pyrrhotite + liquid (or gas) was investigated up to 5,000 bars by means of a new technique. The univariant equilibrium curve for the reaction originates at an invariant point 743 degrees C. and about 10 bars and passes through the points 748 degrees C., 335 bars; 755 degrees , 1,000 bars; 770 degrees , 2,000 bars; and 810 degrees , 5,000 bars. The relations of pyrite in the Fe-S system are deduced from thermodynamic principles and available data. The limitations of the various experimental techniques for sulfide systems are analyzed in the light of the pressure-temperature diagram for the Fe-S system. The apparently large effect of Fe in the vapor or gas of the system is described. The occurrence of primary pyrite in some gabbros, amphibolites, and granites and not in basalts and rhyolites is accounted for by comparing the upper stability curves of pyrite at various partial pressures of S with the beginning of melting curves of rocks of basaltic and granitic compositions under hydrous conditions. When the partial pressure of S is less than about 10 bars small changes in its value have a great effect on the stability of pyrite. The relations of pyrite in the Fe-S system indicate that massive pyrite bodies could not have crystallized directly from a liquid of pyrite composition.