The initial kinetics of the CaO/SO2 reaction have been investigated for reaction times shorter than 1 s and in the temperature interval between 450 and 600 degrees C under both carbonating and non-carbonating conditions (0-20 vol% CO2) to clarify how recirculating CaO influences the emission of SO2 from a modern dry kiln preheater system for cement production. Calcined Faxe Bryozo limestone with a particle size smaller than 400 mu m was utilized as CaO source. It was shown, both theoretically and experimentally, that the observed reaction rates were influenced by mass transport limitations. The results showed that the CaO conversion with respect to SO2 declined when the CO2 concentration was increased. Under all conditions, larger specific surface areas of CaO gave higher reaction rates with SO2. Higher temperatures had a positive effect on the reaction between SO2 and CaO under non-carbonating conditions, but no or even a negative effect under carbonating conditions. The results led to the conclusion that SO2 released from raw meal in the upper stages of the preheater does not to any significant extent react with CaO recirculating in the preheater tower. (C) 2015 Elsevier Ltd. All rights reserved.