Natural oxidation of pyrite in refractory gold bearing rock stockpiles may facilitate the recovery of gold contained in mined low-grade material that was originally sub-economic due to the cost of pre-treatment to oxidise fresh pyrite. The evaluation of this potential requires the oxidation level of pyrite under various conditions. This paper proposes a reaction rate model for prolonged natural oxidation of pyrite under various water saturation conditions and temperatures and a circum-neutral to alkaline pH. The application is to a natural pre-treatment process for refractory gold ore to liberate the gold content encapsulated in pyrite crystals and allow effective cyanidation for gold extraction. The proposed model is based on the shrinking core model with half-order surface reaction on the pyrite surface. It assumes that, in unsaturated water conditions and at circum-neutral to alkaline pH, a diffusion barrier forms on the fresh pyrite surface during the reaction, which prohibits the diffusion of oxygen to the reacting sites and lowers the reaction rate over time. The diffusion barrier may consist of a thin solution film, some precipitations of soluble oxidation products and/or insoluble oxidation products depending on the water saturation and pH. The diffusivity of this diffusion barrier for oxygen is determined as a function of water saturation and temperature. In this model, the concept of effective oxygen concentration is also proposed to represent the change in oxygen concentration at the particle surface due to water; an exponential relationship is found to be appropriate. Based on the reaction rate data reported in the literature, the reaction rate constant in the model for the surface reaction is determined as 5 x 10(-8)mol(0.5).m(-0.5).s(-1) while the diffusion coefficient is 1.2 x 10(-15)m(2).s(-1) for water saturation of 25% and above and 4.7 x 10(-19) m(2).s (-1) for 0.1% water saturation. This rate model can be used for predicting and modelling the level of pyrite oxidation in rock piles or mine tailings where various unsaturated water conditions and circum-neutral to alkaline pH apply.