A kinetic model which describes mass transfer involved in the reaction between a porous solid and a fluid is proposed. The pores are assumed to be spherical and distributed uniformly within the solid particle. The model takes into consideration of rate constant, intraparticle diffusion and specific surface area of the solid which increases as the reaction progresses. From the solution of the differential mass balance for the fluid species, incorporated with the formulation of the specific surface area of the solid, by the finite difference method, conversion of solid versus time is obtained.