In this work, the effects of stoichiometry on phase evolution during the oxidation of mss (monosulfide solid solution) were investigated. A series of mss samples, ranging from Fe7.9S8 to Fe2.37Ni5.53S8 were synthesized from pure components. Samples with grain size 53-90 P m were oxidized at 830 and 850 K in air in a muffle furnace. The Rietveld quantitative phase analysis method was used to identify and quantify the phase information from powder X-ray diffraction (XRD) profiles. Hematite was observed and accounted for most of the oxidized iron. Nickel in mss was not oxidized to NiO under current isothermal conditions; instead, it was finally transformed to Ni17S18. Hematite, Fe-2(SO4)3 and residual mss were identified in the final phases after 24 h oxidation of the mss composition Fe7.9S8; hematite and Ni17S18 for compositions Fe6.15NI1.54S8 and Fe2.37Ni5.53S8; hematite, Ni17S18 and pentlandite for Fe6.4N1.6S8. Given a constant iron to nickel atomic ratio of 4: 1, the sample with lower metal concentration, Fe6.15Ni1.54S8, showed a faster oxidation rate than its metal richer counterpart, Fe6.4Ni1.6S8. The mean oxidation rates for these two samples are 1.85 x 10(-4) and 1.22 x 10(-4) s(-1) respectively for 1.5 It heating at 830 K. Vyazovkin's theory of changing activation energy (E-a) with reaction extent (y) was employed in the current kinetic study. The activation energy was determined using a model-free method. The oxidation of Fe6.4Ni1.6S8 exhibited a higher E-a than Fe6.15Ni1.54S8 over the course of reaction. The activation energy increases with y from 67.1 to 103.3 U mol(-1) for mss composition Fe6.15Ni1.54S8; 76.1 to 195.0 U mol(-1) for Fe6.4Ni1.6S8. Bulk compositions Fe7.9S8, Fe2.37Ni5.53S8 Were selected to give a constant metal to sulfur atomic ratio of 7.9:8. Oxidation of Fe2.37Ni5.53S8 achieved equilibrium within 1 h, compared to 5 h for Fe7.9S8. (C) 2004 Elsevier B.V. All rights reserved.