A pyrite concentrate with a grind size P-80 of 14 mu m was oxidized in an alkaline medium at an oxygen overpressure between 10 and 40 psig, at a temperature of 80 degrees C. Sodium hydroxide was used to neutralize the acid produced as a result of pyrite oxidation. When the pH was maintained above 12.5, there were three dominant sulphur oxyanions detected in the solution, thiosulphate, sulphite and sulphate. The focus of this study was the calculation of thiosulphate yield as a function of sulphide sulphur concentration of the pyrite in the feed. This calculation included the effect of other sulphur oxyanions on the thiosulphate yield since it is well known that thiosulphate oxidizes fairly rapidly to tetrathionate and trithionate and higher sulphur oxyanions under high alkalinity conditions. The transient formation of tetrathionate and trithionate can only be estimated when the mechanism of formation of these oxyanions is known. Various schemes involving branching of the reaction pathways can be found in the literature but unfortunately, no widely accepted mechanism has emerged from these studies. In the present study, the transformation mechanism is based on the least electron transfer principle and assumes that the transformation reactions occur in series. Based on this approach, linear equations were derived and used to set up a kinetic model of the system with respect to thiosulphate yield. In determining the kinetic constants, the presence of unseen and unmeasured intermediates was assumed and calculated based on the corresponding reaction stoichiometry. A single rate expression combining kinetic constants of all the metastable oxyanions was derived to predict the thiosulphate yield as a function of the known pyrite sulphide sulphur concentration under the experimental conditions that were adopted. dCs203/dt = 2(-1)k(1) k(2)[C(o)s(-2)][Po-2](proportional to 1) [OH](beta 2){1-k(4) [Po-2](proportional to 4) + 2(-3)k(4)k(5)[Po-2](proportional to 4)[OH](beta 5) (5 + k(6)[OH]beta(6))} (C) 2015 Elsevier Ltd. All rights reserved.