To better understand the sulfur capture process during fluidized bed combustion, the reaction between sulfur dioxide and limestone has been investigated under periodically changing oxidizing and reducing conditions. All experiments were carried out in a fixed-bed quartz reactor at 850 degrees C, with the reactant gas consisting of 1500 ppm SO2, 10% CO2, and with the O-2 and CO concentration varied between 0 and 4%. The fraction of time under reducing conditions was varied between 0 and 100% and the total cycle time (i.e., time of one oxidizing period + one reducing period) was varied between 30 and 900 s. The present study showed that alternating conditions may result in both an increase and a decrease in the final conversion depending on both the total cycle time and the fraction of time under reducing conditions. Whereas a final conversion of approximately 9% was found when the limestone was sulfated under oxidizing conditions, conversions between 1% and 25% were found under alternating conditions for the cases where only small amounts of CaS was formed. For the experiments where most of the sulfur was in the form of sulfide the conversion was higher, up to 46%. The conversion increased for all cycle times up to 33% of the time under reducing conditions, with the largest increase seen for long cycle times. When the fraction of time under reducing conditions was increased further, the conversion decreased significantly for the shorter cycle times.