The combustion performance of two samples of biomass fuel were evaluated using a small scale 50 kW laboratory scale stoker type combustor. One fuel was a bark residue of moderate moisture content and rich in aluminosilicate inorganic contaminants from the surrounding soils. The other was stemwood from a purpose grown eucalypt E. nitens. It had a high moisture and was rich in bio-accumulated inorganics. such as calcium and potassium. During combustion the inorganics underwent a complex set of transformations. Some of the major reactions were identified and shown to have important consequences in terms of overall combustion performance as judged by such parameters as extent of ash slagging and fouling, ash leaching characteristics, trace element and sulphur emission levels.