Particle size distribution (PSD) analysis of the Gregory mine coal rejects, over a period of time is presented. The PSD has important implications on several processes contributing to the acid mine drainage. In this paper, correlation between the particle breakdown in the coal rejects and the rate of oxidation of sulphides is presented. Difference between the initial and the subsequent values of particle size distribution coefficient over a period of time indicated a change in particle breakdown mechanisms. An initial physical breakdown due to structural breakdown/rearrangement caused by wetting of the sample, and subsequent breakdown caused by the combined effects of physical and chemical weathering are proposed. Total number of particles increased with time, as the mean size of the distribution decreased. Increase in surface area calculated form the PSD analysis followed a power law with respect to time. The pyrite oxidation rate is dominated by diffusion and mass transfer processes initially, and as the total surface area is increased, contributed by breakdown of larger particles, a phase boundary control dominates the oxidation process. (c) 2006 Elsevier B.V. All rights reserved.