The reaction between limestone and SO2 was investigated under both oxidizing and periodically changing oxidizing and reducing conditions at atmospheric pressure in a fixed-bed quartz reactor. Three limestones of different reactivities were sulfated at temperatures between 800 degrees and 875 degrees C with a gas mixture of 1500 ppm SO2, 10% CO2 and alternating O-2 and CO (0% or 4%). The experimental data found that periodically changing oxidizing and reducing conditions could have either a positive or a negative effect on the degree of conversion of CaO to CaSO4 compared to limestone samples sulfated under constant oxidizing conditions. In the temperature range 825-875 degrees C the conversion was relatively constant for all three limestones under oxidizing conditions. However, when the limestone samples were sulfated with periodically changing oxidizing and reducing gas mixtures a wide range of conversions were found. The highest degree of conversion was found at 825 degrees C for all three limestones and the lowest degree of conversion was found at 875 degrees C. The rapid decrease in conversion at high temperatures was due to the rapid release of SO2 due to both the decomposition of CaSO4 as well as a high rate of CaS oxidation. A comparison of the sulfation rates of several experiments performed with Koping limestone under alternating conditions showed a wide range of rates, indicating the difficulties in using laboratory rate data in sulfur capture modelling for fluidized-bed boilers.