Variation of the reactivity of a solid reactant due to changes in number of working active sites and pore structure was represented by a deactivation model. The model was shown to give good aggreement with the experimental data obtained for the gasification of chars of different lignites. All the kinetic data obtained at different temperatures (800 degrees-950 degrees C) and also for different types of coals were represented with a single generalized relation. The increase of the surface area of the char at the initial stages of gasification did not cause a corresponding increase in the reaction rate. Results indicated that the surface was composed of highly reactive and less reactive sites. It was shown that the reactivity of unit surface area of the solid reactant first showed a rapid decrease due to decrease of number of more reactive sites at the initial stages and then reached to about one fourth of its initial value.