Limestone dissolution in acid solutions was experimentally studied by means of a fixed-bed reactor. A diffusive model was proposed to describe experimental results. The model, based on the film theory, takes into account the effect of the electric potential of diffusion on the dissolution rate. In order to validate the model, experiments were carried out to study the effect of both particle size and liquid-solid relative velocity on the dissolution rate. Furthermore, the limestone dissolution rate in aqueous solutions of SO2(aq), HCl, and H2SO4 was measured at different acid concentrations. The results showed that the diffusive model, associated with the correlation proposed by Chu et al. [Chu, J. C.; Kalil, J.; Wetteroth, W. A. Chem. Eng. Prog. 1953, 49 (3), 141] for the evaluation of the liquid-solid mass-transfer coefficient, is capable of describing the dependence of the dissolution rate on both the size of the limestone particles and the liquid-solid relative velocity. In addition, it was found that, in agreement with the physical meaning of the film theory, the thickness of the liquid film is not dependent on the kind of acid present in the solution nor on its concentration but only on the fluid dynamic conditions of the liquid-solid system.