Heat treatment of (Fe,Ni) (sub 1-x) S solid solutions, equivalent to some nickeliferous pyrrhotites is shown as giving rise to oriented blebs, stringers and rims as well as apparently cross-cutting "veins" of pentlandite in pyrrhotite. As the process must involve diffusion of iron, nickel and sulfur these intergrowths and segregations of pentlandite are attributed to exsolution following particularly loss or readjustment of sulfur between pyrrhotite and pentlandite, as indicated in the latter case by Kullerud. Fracturing and late introduction of chalcopyrite are suggested as factors aiding such ex-solution in natural ores. Some of the products illustrated resemble natural occurrences but others do not. "Veins" of pentlandite are thus not necessarily due to crystallization from a liquid of this composition.