Sulfur capture by limestone under periodically changing oxidizing and reducing conditions was investigated in a quartz reactor. To simulate conditions experienced by limestone particles in fluidized bed combustion, short periods of oxidizing and reducing conditions were used. Total cycle times (time under oxidizing conditions + time under reducing conditions) between 2 and 24 s were used. Furthermore, three different fractions of time under reducing conditions were tested: 50, 33, and 25%. The degree of conversion of CaO to CaSO4 under these conditions is very dependent on both total cycle time and fraction of time under reducing conditions. For a fraction of time under reducing conditions of 50%, a minimum in the conversion (1.9%) versus the total cycle time was found at 8 s. For shorter cycle times a dramatic improvement: of the conversion was obtained, e.g,, 14.2% for 6 s. These high conversion values were also 50% higher compared to those obtained for the same Limestone Sulfated under totally oxidizing conditions. When the fractions of time under reducing conditions were 33 and 25%, alternating conditions have a positive effect on the conversion compared to that obtained under oxidizing conditions for all cycle times.