The aim of this paper was to gain a better understanding of the sulphur capture process during fluidised bed combustion (FBC). To achieve this the reaction between limestone and SO, was investigated under periodically changing oxidising and reducing conditions using a fixed-bed quartz reactor. The oxidising gas consisted of 1500 ppm SO2, 10% CO2 and 4% O-2. Three reducing gas mixtures were evaluated: (i) 1500 ppm SO2, 4% CO2 10% CO2, (ii) 1675 ppm H2S, 4% CO2 10% CO2, and (iii) 1675 ppm H2S, 4% H-2, 10% CO2. The limestone samples were sulphated for 2 h at 850 degrees C with a total cycle time, i.e., one oxidising period followed by one reducing period, of 30-900 s, and with the fraction of time under reducing conditions in the range 0-67%. Alternating conditions had either a positive or negative effect on the final degree of conversion compared with samples sulphated under constant oxidising conditions. For cycle times of 30-240 s, a conversion maximum was found when the fraction of time under reducing conditions was in the range 20-50%. The location and extent of this maximum were dependent on the gas mixture used and the cycle time, When using a cycle time of 900 s, the degree of conversion increased with an increasing amount of reducing conditions. The highest degree of conversion was obtained using H2S/H-2 in the reducing gas, followed by H2S/CO and SO2/CO. Under alternating conditions the final degree of conversion was 1-33%, compared with a conversion level of 9% when the same limestone was sulphated under constant oxidising conditions. Thus, alternating conditions may not only explain why a poor sorbent sulphation is sometimes found in utility fluidised bed boilers (FBB) but also why a surprisingly high degree of sulphation is sometimes obtained.