A reaction model has been developed for the estimation of diffusivity of the intercalated species in graphite on the basis of a material balance expressed in terms of cylindrical coordinates, where the species in the layered structural cylinder diffuse radially from the center to the surface and react with gas. The resultant kinetic equations are used to analyze the experimental data for the oxidation rate of potassium atoms intercalated in natural graphite powder of initially different sizes intercalated at the first stage.