A model is developed for pore diffusion controlled gas-carbon reactions where the reaction is limited to a narrow region on the exterior surface of the carbon particle. A planar geometry is used and an isothermal condition is assumed. However, the effects of pore structure change with conversion are included. The model can predict the penetration depth, the porosity profile, the reaction surface area profile and the concentration profile in the reaction zone. The model is an extension of Gavalas model[1] and an improvement over Desai and Yang’s model[2]. The model predictions are compared with available experimental data and with those of Desai and Yang’s model.