The effects of the presence of a Pt catalyst on the limestone/lime sulphation process were investigated by thermal analysis methods to provide a better understanding of the factors limiting gas desulphurization when Ca-based sorbents are used. It was found that for the Pt-catalysed sulphation of precalcined limestone the weight increase is above 100% higher under isothermal and dynamic conditions (up to 830°C). These results are direct evidence that Pt catalyses the CaO-SO2-O2 reaction. It can be presumed that the process proceeds through a gaseous intermediate, SO3, a highly reactive gas, which explains the increased rate of sulphation. SO3 then reacts with CaO to form CaSO4 directly, in contrast with the non-catalysed oxidation of SO2 to SO3, where CaSO3 formation is the most probable early stage of sulphation. The proposed mechanisms were supported by the phase identification of the products.