Theoretical and experimental studies were conducted on coal sulphur removal under chaotically changing oxidizing and reducing conditions, which occur in the dense zone of bubbling fluidized beds. Experimental desulphurization trends were analyzed by comparison with multi-species (50 gas-phase, 7 solid-phase species) equilibrium calculations for coal/steam/sorbent/air system. It was shown, that under fluctuating reducing/oxidizing conditions, sulphur capture as CaS in locally reducing zones increased the sulphur retention level of the sorbent. In experiments, performed in a 8 cm i.d. bench-scale FBC boiler, effective sulphur retentions were obtained in the 800-1000 degrees C temperature range. As a chemical interpretation, a time history of sorbent particle calcination-sulphation is proposed, which leads to the formation of a particle structure with a sulphided (CaS) core and a sulphated (CaSO4) shell. Full-scale applications are discussed and measures for improving limestone utilization and high-temperature sulphur removal in fluidized beds are recommended.