The active surface area progression which takes place in natural graphite flakes during oxidation is difficult to model analytically. A probability based simulation was developed to represent the observed behaviour. The simulation is simple and capable of easily representing complex geometries such as randomized structures or curved contours which would be difficult and time consuming to model analytically. The probability used in the simulation is directly linked to the kinetic parameters. These parameters and conversion behaviour were experimentally measured for a high purity, flake natural graphite sample. In addition, the sample was partially oxidised and extensively examined under a scanning electron microscope. Three characteristic microstructures were identified in the oxidised sample. These structures were qualitatively approximated by three simulated structures. Two of the three simulations were unable to effectively represent the initial region of the conversion function where it increases towards a maximum. The complete behaviour was accurately represented by the simulation of a square flake with a few randomly positioned pits growing in the flake structure. The simulation illustrates that the initial increase is due to the growth of the pits within the larger flake structure. The pits were found to be caused by catalytic impurities. Thus by demonstrating this is a rate controlling factor the simulation enables the practical insight that the oxidation resistance can be significantly improved if these small impurities are removed. (C) 2013 Elsevier Ltd. All rights reserved.