The concentration of sulphur compounds inside the combustion chamber of a 12 MW circulating fluidised-bed boiler was measured at different operating conditions: (a) bed of sand without limestone addition, and (b) three cases of air-staging in the presence of limestone. The data obtained with a sand bed show that most of the sulphur is released in the dense bed. In presence of limestone, a high concentration of SO2 was seen below the secondary air inlet, when either normal or intensified air-staging were used. In the same area reduced sulphur compounds, 300 ppm, were found under intensified air-staging. (C) 2001 Elsevier Science Ltd. All rights reserved.