Two bark/wood chips fuels and one pelletized straw fuel were studied with regard to elemental composition, ash composition and sintering/melting behaviour with an emphasis on the chemical reactions involved in sintering. Ashes were prepared by two different methods: low-temperature laboratory ashing and combustion in a laboratory fixed-bed furnace. X-ray diffraction, thermogravimetric analysis and scanning electron microscopy indicated the type of crystalline compound which can be found in ashes from biomass fuels with a high calcium, silicon and potassium content. The potassium content of the straw ash was 3 times higher than that in the bark/wood fuels. This potassium was present in crystalline compounds with low melting points and in the molten phase of sintered straw ash.