This paper presents selected results from an extensive on-site measurement campaign where the ash behaviour in a 12 MW CFB boiler was studied during firing of coal, peat and wood. Samples were taken from all in-going (bed material, fuel) and out-going solid material streams (secondary cyclone and bag filter) as well as from the bed and the return leg. Deposit samples were further collected from the cyclone inlet and from two different locations in the convective path. In addition, the boiler operation was monitored, including collection of operational data, flue gas temperature profiles and emissions. The paper discusses the differences in the ash chemistry that were detected between the three different combustion cases and draws conclusions on the impact of the chemistry on the bed agglomeration and fouling tendency for each fuel.