The oxidation of coals of different rank, origin and particle size has been studied at temperatures between 423 and 543 K and for time intervals between 6 h and 42 days. The chemical composition of the oxidised coal depends upon the coal rank, its particle size and the degree of oxidation as determined by the temperature and the time of oxidation, At higher degrees of oxidation, whether at higher temperatures for shorter time intervals or at lower temperatures for longer time intervals, the oxidised coal tends to approach similar chemical compositions. The weight of coal also changes on oxidation, the increase or decrease in weight depending upon the rank and the oxidation conditions. The rate and extent of oxidation decrease with increase in particle size because the larger particles slow down the diffusion of oxygen into the coal particles. The oxidation can eliminate completely the plastic properties of bituminous coal which inhibit the formation of anisotropic structures and enhance the development of a primary pore structure. The helium density increases with the degree of oxidation but the mercury density initially increases and then decreases. A minimum in the mercury density is obtained when the carbon content of the oxidised coal is around 85-90%. The oxidation significantly enhances the porosity and the surface area, the extent of increase depending upon the nature of the coal and the degree of oxidation. The changes in chemical composition, porosity and surface area with the degree of oxidation indicate that the oxidation of coal involves two different mechanisms, one operating at lower temperatures and the other at higher temperatures.