A mathematical model to analyse the internal combustion of a porous carbon particle is presented. The carbon particle is modeled as a porous sphere with uniform spherical pones arranged on a simple cubic lattice extending throughout the particle volume. During combustion, oxygen diffuses into the particle's pores, is adsorbed at the pore walls and reacts isothermally with the carbon producing mainly CO 2. The CO 2 is desorbed and diffuses towards the surface of the particle countercurrently to the oxygen. As the reaction proceeds the pores enlarge and eventually merge, causing the partial coUapse of the particle. The rate of pore enlargement is not uniform throughout the particle. It is dependent on the extent of oxygen penetration which is determined by the particle size and the prevailing temperature regime. When the pores intersect, the carbon of the pone walls collapses into spherical units equal in number to the intersectinF pores. The chemical rate expression used is based on an adsorption-desorption mechanism of the Langmuir-Hinshelwood type proposed by Essenhigh, Froberg and Howard in their reanalysis of Tu, Davis and Hottel data . The model is used to examine the effect of particle size, temperature and extent of burnoff on the rate of combustion and change in internal specific surface area. The surface area changes obtained are consistent with experimental observations reported in the literature. Data from the literature on the combustion of single carbon spheres, soot and pulverized semianthracite are compared with those obtained from the model using Tu et at. data. Although the combustion of various types of carbon have the same activation energy, they differ widely in reactivity at the same temperatune~. The model shows that the different reactivities cannot be accounted for by diffusion, either internal or external to the particle. indicating that the differences are most likely due to the intrinsic chemical properties of each carbon type.