The reductive decomposition of CaSO4 was studied in a 12 MW circulating fluidised-bed boiler. Measurements were made inside the combustion chamber under normal operating conditions, except that the bed material contained old, sulphated, sorbent. Addition of limestone was stopped two days before the tests, in order to eliminate the effects of fresh sorbent. Gas concentrations were measured in the centre of the riser cross-section at different heights at two bed temperatures 850 and 920 degrees C. At one height, measurements were also made in nine positions distributed over the horizontal cross-section. The data were compared with results obtained with a sand-bed. The results show that reductive decomposition takes place below 0.5 m height (that is, in the dense bottom bed of the combustion chamber) and that most of the released sulphur is recaptured in the splash zone immediately above the dense bottom bed. This means that most of the recapture occurs below the secondary air ports, indicating the important role of the dense bottom bed in the sulphur-capture process.