Sulphur capture by limestone was investigated in a quartz reactor under periodically alternating oxidising and reducing conditions, in order to simulate the environment in a fluidised-bed combustor. The influence of the SO2 concentration was studied. The lower SO2 concentration was, however, compensated for by longer test periods; thus the product [SO2] x (total test period) was constant. The inlet SO2 concentration was varied between 100 and 1500 ppm and the total test periods were 2 to 30 h. Total cycle times (oxidising + reducing time period) from 2 to 720 s were used. For cycle times shorter than similar to 6 s, the conversion to CaSO4 hardly changed with the inlet SO2 concentration. Furthermore, alternating conditions gave higher conversions than under totally oxidising conditions. For long cycle times, however, the conversion sharply decreased with a decrease of the SO2 concentration and, for low SO2 concentrations, the conversion under alternating conditions was smaller than under oxidising conditions.