The oxidation of iron and nickel sulphide concentrates, and the dominant sulphide minerals which comprise them, has been studied under ignition conditions using thermogravimetry (fG). The concentrates were derived from various minesites and they showed differing tendencies to ignite. Ignition temperatures ranged from 416 to 488C and the extents of reaction, measured at 500C, varied from 64 to 88 percent. The reactivity of the least reactivematerial was improved significantly by grinding. Particles <63 lm showed 85 percent oxidation compared with 55 - 65 percent for particles>63/lm.Examination of the individual sulphide minerals revealed that these materials also have significantly different degrees ofreactivity and are subject to particle size effects. For particles <45 lm, the ignition temperatures were as follows: pyrite and violarite 380 - 385c; pentlandite 425 - 430c; and pyrrhotite 470 - 475; for particles>90 /lm, the ignition values were: pyrite 395 - 400C; violarite 430 - 435C; pyrrhotite 540 - 545C; and pentlandite 560 - 565C.Using the data on the individual minerals it was possible to rationalise the differing degrees of reactivity of the concentrates. The role of the most reactive constituent in initiating the ignition reaction has also been discussed.Finally, the results of this work were found to compare. favourably with the behaviour ofthe concentrates in industrial and pilot-scale flash smelters.