A total of 19 soils collected from the agricultural areas of Western Australia were tested in an incubation experiment for ability to oxidize pyrrhotite (Fe7S8). Oxidation occurred in all soils and was apparently biological as it was inhibited by mercuric chloride. In eight of these soils the effect of supplying phosphate on oxidation of pyrrhotite was compared with the effect on oxidation of elemental sulphur. Phosphate stimulated oxidation of sulphur but in six of the soils it depressed oxidation of pyrrhotite. In another incubation experiment, the rate of oxidation of different size gradings of pyrrhotite and pyrites was compared with that of elemental sulphur. Fine particles of pyrrhotite oxidized at a similar rate to fine particles of sulphur and about four times as rapidly as fine particles of pyrites. Coarse particles oxidized more slowly than fine, but the effect was least marked with pyrrhotite. Pyrrhotite, pyrites, and elemental sulphur were compared with sulphate as sources of sulphur to plants grown in pots. Pyrrhotite and elemental sulphur were effective sources but pyrites, applied at equivalent rates, produced only small responses. Waterlogging decreased the response to sulphate and to elemental sulphur, but not the responses to pyrrhotite. Effects of pyrrhotite were measured in field trials at Darkan and Pinjarra and effects of pyrites at these sites plus Kwolyin. Pyrrhotite produced significant increases in yield of dry matter and yield of nitrogen at Darkan and in yield of nitrogen at Pinjarra. No significant responses to pyrites occurred in the year of application, though there were large responses to elemental sulphur applied at equivalent rates.