A method of analysis is presented for the reaction between a porous solid and a gas when the diffusion resistance is significant. The kinetic expression used is linear with respect to the concentration of reacting gas. As a typical example, experiments of gasification of graphite sph-res with carbon dioxide were carried out and the results were compared with the proposed model, where the change of the porous structure due to the reaction is considered. The computed values agreed favorably with the experimental data for the early stage of the reaction.