Results are presented on the distribution of some (inorganic) elements between the solid and vapor phases in a circulating fluidized bed boiler during combustion of wood chips and coal, Samples were taken from all input and output solid material streams as well as from the gas phase at different locations in the boiler. Deposits on a specially designed deposition probe were also measured. An explanation is presented concerning the release and fate of potassium, chlorine, and sulfur during biomass combustion. The explanation is supported by the results from an additional test where hydrogen chloride was added to the combustion chamber. One of the most important factors affecting the reactivity of potassium toward chlorine in the combustion chamber appears to be the moisture content of the fuel.