Despite the rapid technical development of circulating fluidised bed (CFB) combustion, knowledge of the detailed combustion chamber processes is insufficient as most attention has been given to the overall performance of the boilers. However, knowledge of the detailed processes is necessary for further improvement of the technique. The present work focuses on the combustion chamber processes and, in particular, on the bottom part. The question to be addressed concerns the properties of the bottom zone and the existence of a dense bottom bed. In the cases studied, the 600 MWth CFB boiler in Gardanne, France, and the 12 MWth CFB boiler at Chalmers University, dense bottom beds were found with bed material moving predominantly upwards in the centre and downwards at the walls. The behaviour of the bottom beds in the two boilers was similar in most respects, but the measurements in the larger boiler revealed, in contrast to the fluctuation between oxidation and reduction in the 12 MW boiler, constantly oxidising conditions at the measurement position, which are most likely related to insufficient dispersion of bed material across the 35 m(2) cross-section.