A numerical model for simulating a dry forward in-situ combustion process in two dimensions, was developed. The primary focus is on the simulation of the compositional changes that take place inside the porous medium during the running of the process. The model allows any number of hydrocarbon components and six others, namely, liquid water, water vapour. oxygen, nitrogen, carbon dioxide, and carbon monoxide. It describes the flow of water. oil, and gas, and includes the gravity and capillary effects. The vapourisation and condensation effects of both hydrocarbons and water enhanced the heat transfer, primarily by conduction and convection, ahead of the combustion front. Equilibrium calculations are performed on the components in both the liquid and vapour phases. The changes in pressure, temperature, and flUid compositions govern the direction of the interphase mass transfer. Heat is generated by two types of reaction, namely, low temperature oxidation and burning of the crude oil. The model allows the movement of a thin burning front inside the burning cell. It is found to give a better temperature profIle. representative of the combustion process. Oxygen mole fraction is calculated throughout the porous medium according to the reaction kinetics. thus no assumption is made regarding the degree of oxygen consumption. The effects of oxygen bypassing caused by the kinetic-limited combustion is therefore represented. A total of 18 components were used in the computer runs. Results show the preferential vapourisation of the lighter components in the vicinity of the high temperature burning front. The lighter components then move towards the producer. faster than do the heavier ones. This segregation produce fuel that is heavier than the original oil. High temperature in the upstream cells causes a reduction in the oil viscosity. which in turn increases its mobility, thus transporting more heat downstream. The rise in temperature in the condensation cell results in a decrease in the rate of water vapour condensation; extending the condensation zone downstream. In the high pf(>ssure run. all the hydrocarbon in the downstream cells condenses. In the burning cell however. both the vapour and the liquid phases are present due to the high front temperature. The vapour phase is richer in the light components while the liqUid is richer in the heavy components.