The reaction of SO2 with Ca(OH)(2) to form a solid product is important for SO2 removal processes. After the partly reacted calcium sorbent is withdrawn from the SO2 containing flue gas, it is re-injected up-streams in the flue gas duct to enhance the solid conversion by increasing the residence time. Besides, the reactivity of the partly used sorbent towards SO2 is enhanced i.e. activated when the sorbent is temporarily exposed to a gas with high water content leading to a much higher conversion. In laboratory fixed bed experiments the increase of the reactivity of the sorbent is investigated by consecutively exposing the sorbent to a SO2 containing flue gas and a gas with a high water content. Based on a pseudo-kinetic model the fixed bed reactor is mathematically simulated and only one model parameter is adjusted which describes the impact of the activation on the consecutive desulfurization. The reactivity is significantly enhanced which depends upon the conversion reached hitherto. The activation can be the explanation for the surprisingly good SO2 removal efficiency of dry low temperature desulfurization processes, where liquid water is sprayed into the flue gas duct where it evaporates and provides locally a high relative humidity.