The sulphation reaction of hydrated silica fume-Ca(OH)(2) sorbents were investigated and modelled. These experiments were conducted in a thermogravimetric analyzer under isothermal conditions and in a humidified gaseous atmosphere consisting of 5 vol% O-2, 10 vol% CO2, 0.5 vol% SO2 and the balance of nitrogen. The effect of the relative humidity on the reaction rate was determined by sulfating of hydrated sorbents in the gaseous atmospheres which have three different relative humidities: 25%, 55% and 90%. The unreacted shrinking core model was chosen to describe this non-catalytic solid/gas (hydrated sorbent/SO2) reaction mechanism. The experimental results were found to be correlated successfully by this model. It was determined that chemical reaction is partly rate limiting especially at the beginning of sulphation, diffusion through the calcium sulphite hemihydrate product layer generally controls the reaction rate.