Four samples of violarite of different stoichiometry were synthesised and characterised. The pyrolytic decomposition of Fe0.97+/-0.01Ni1.94+/-0.02S4.0+/-0.03 of particle size 45-63 mu m was examined at a heating rate of 10 degrees C min(-1) in a nitrogen atmosphere using a simultaneous thermogravimetric-differential thermal analyser (TG-DTA). The first reaction observed was the loss of sulfur between 420 degrees and 525 degrees C, giving a mass loss of 2.25% with the formation of a monosulfide solid solution (mss), violarite of a different stoichiometry, and a minor amount of vaesite. An endothermic peak at 495 degrees C was assigned to a phase transition from a thiospinel structure to mss. Further sulfur losses of 7.65 and 4.10% took place in the 525-615 degrees C and 615-795 degrees C ranges, and the products were mss of increasing metal richness, with (Fe,Ni)(3+/-x)S-2 also existing in the higher temperature range. This last phase melted at 835 degrees C. Finally, more sulfur was lost between 845 degrees and 900 degrees C from the molten material. Products were taken at different temperatures for characterisation by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The TG-DTA curves for all four samples were obtained under similar conditions. The samples were all found to decompose by the same reaction sequence. The endothermic events associated with the thiospinel transition, and the melting of (Fe,Ni)(3+/-x)S-2, shifted to lower temperatures as the nickel concentration of the original violarite increased.