Sulfation experiments were carried out and product layers were characterized to investigate the reaction kinetics and to test a Crystallization and Fracture (CF) model describing the: mechanism of formation of product layers. Sintered samples of calcined Limestone were sulfated in a quartz reactor for short and extended times in order to obtain initial and fully developed product layers. The product layers were characterized by means of Scanning Electron Microscopy (SEM) visualization and measurements of BET surface area and pore size distribution. At lower temperatures, the sulfation reaction ceased after a relatively shea time, whereas at higher temperatures, the reaction continued at an approximately constant rate for as long as 30 h. The reaction product was formed by crystallization. The product "layer" formed in the early stages of the reaction was not a true layer, but isolated nuclei and crystals. The "continuous" product lever formed in the later stages of the reaction was found to be a monolayer of individual crystals with pores of size 20-30 Angstrom along the boundaries. The product layer was more porous when developed from larger stable nuclei formed during the initial reaction at higher temperatures and lower SO2 concentrations. These observations support the mechanism described by the CF model.