A simulation model is developed to analyse the dynamic behaviour of the SO2-limestone reaction under periodically changing oxidizing/reducing conditions, Reference conditions assumed in the model are those typical of atmospheric fluidized-bed combustors operated with sorbent addition. The reaction network embodies limestone calcination, lime sulphation and sulphate reduction by carbon monoxide, relevant to periodic establishment of mildly reducing atmospheres around the sorbent particle. The model applies to the early conversion of particles and for short periods of time within the sulphation time. The influence of operating conditions on the rate of sulphur uptake by the sorbent is assessed, with particular emphasis on the effect of the cycle time of periodic shift between oxidizing and reducing conditions. Reaction regimes corresponding to the limiting cases of very short and very long cycle times are characterized. Analysis of model results is accomplished in the light of time-scales of circulation and periodic exposure to oxidizing/reducing conditions experienced by sorbent particles in fluidized-bed combustors.