A mathematical model for describing transport, reaction, and pore structure evolution during gas—solid reactions with solid product is modified and extended for application to porous media whose structure can be represented by a network of pores of distributed size and length. Model equations are presented for both densification and gas—solid reactions with solid product. The effects of nonuniform pore length on the model predictions are examined by using the model to describe the sulfation of porous CaO and calcined limestones. We also investigate the dependence of the model predictions on the formula used to estimate and follow the evolution with the conversion of the effective diffusivity of the gaseous reactant in the pore space.