Three different structural models, a random pore model, a changing grain size model, and a distributed pore size model (DPSM), have been used to predict the sulfation conversion (X(s)) vs time (t) curves of seven Limestones with different pore structure in seven narrow particle size cuts between 100 and 2000 mu m. None of the models were found able to predict the experimental results X(s)-t with all the particle sizes considered. The results were improved by changing the relation of the effective diffusivity in the pores with conversion and, specifically, the change of tortuosity with porosity. Thus, the results of this study showed that the values of the exponent b in the equation to calculate the tortuosity had to be determined for each particle size, except for the DPSM model with limestones with widespread pore size distribution. All three models give almost identical results for the highest particle sizes using the product layer diffusion coefficient D-s and the exponent b of the tortuosity equation as characteristic of each limestone.