In pressurised fluidised bed combustion, sulphur dioxide is bound directly to uncalcined limestone. Diffusion through the calcium sulphate product layer has been found to be important in limiting the sulphation rate. The product layer diffusion controlled regime of the sulphation was studied in a pressurised thermogravimetric apparatus. The apparent activation energy of the product layer diffusion has been earlier reported to be high, of the order of 100-200 kJ/mol. The purpose of the work was to study more closely the cause of the high observed apparent activation energy. Experiments were made to decouple the effect of temperature on the diffusion process itself from the possible effect of the temperature on the type and structure of the product layer. The high values of the apparent activation energy were found to be due to the diffusion process itself. The diffusion process was dependent on the gas phase SO2 concentration raised to the power of 0,49±0,04. Both of these results indicate that the diffusion through the product layer occurs via solid state diffusion. Scanning electron microscopy photographs taken of partly sulphated limestone particles supported the assumption that the sulphation of uncalcined limestone proceeds as a shrinking core process.