Pentlandite oxidation behavior in the temperature range 530–600°C was investigated by microscopic and kinetic methods. A clear four-step reaction sequence was identified through monitoring the reaction progress by Rietveld quantitative phase analysis of quenched samples. The first step involves the transformation of pentlandite to monosulfide solid solution (mss), (Fe,Ni)S. This is followed by the transformation from mss to α-NiS in the second step, the oxidation from Fe 3 O 4 to Fe 2 O 3 in the third step, and the oxidation of α-NiS to NiO in the final step. Avrami/Arrhenius analysis indicated rapid transformation of pentlandite to mss with rate constant increasing from 3.8×10 −4 s −1 at 530°C to 2.1×10 −3 s −1 at 600°C and an activation energy of 140kJmol −1 ; the rate constant for the transformation from mss to α-NiS increases from 6.7×10 −5 s −1 at 530°C to 4.4×10 −4 s −1 at 600°C with an activation energy of 151kJmol −1 , and for the oxidation from Fe 3 O 4 to Fe 2 O 3 increases from 3.2×10 −5 s −1 at 530°C to 1.7×10 −4 s −1 at 600°C with an activation energy of 127kJmol −1 . Such clear reaction mechanism and the associated kinetic data can be applied to optimize the operation conditions in industrial rosters and flash smelters to improve process efficiency and Ni recovery.