The phase relations of pentlandite assemblages in the Fe-Ni-S system have been reinvestigated by sealed, silica tube annealing experiments at 600 degrees , 500 degrees , 400 degrees , 300 degrees , and 230 degrees C. The compositions of the coexisting phases in the quenched products have been determined by electron probe analyses, so that the phase boundaries are located with much greater accuracy than in previous work and actual tie lines plotted. The tie lines between pentlandite solid solution (pns) and monosulfide solid solution (mss) change direction at a mss composition of about 33 at. % Ni, correlating with the composition at which the mss field breaks down below 400 degrees C. The mss field, continuous at 400 degrees C, extends only to about 25 at. % Ni at 300 degrees C; the other more S-rich phases coexisting with pns at this temperature being a second mss phase, containing about 33 at. % Ni, and beta (Ni,Fe) S containing about 5 at. % Fe. The mss field withdraws to 17 at. % Ni at 230 degrees C, enabling pyrite-pns tie lines to be established. At 230 degrees C, the limits of solid solution of Ni in pentlandite are 15 at. % when coexisting with troilite and 34 at. % when coexisting with beta (Ni,Fe) S.Electron probe analyses have been made on a variety of natural pentlandite assemblages. A compilation of data from all sources confirms that the composition of pentlandite varies systematically with the assemblage, ranging from 18 at. % Ni in the assemblage troilite-pentlandite to 34 at. % Ni in the assemblage millerite-heazlewoodite-pentlandite. The Ni content of pyrrhotite and the Fe contents of millerite and heazlewoodite are significantly lower than the corresponding data for the synthetic system at 230 degrees C, suggesting that chemical readjustment persisted to very low temperatures in these assemblages. The good agreement among several limiting compositions in comparing the synthetic and natural data indicates that the principal factors controlling the phase chemistry of natural nickel sulfide assemblages are related to crystal-crystal equilibria.