Improved knowledge of the melting and gas-phase release behavior of ash from biomass fuels is of premium importance, to reduce ash-related problems in straw-fired boilers. In this study, a simultaneous thermal analysis (STA) instrument was used to characterize the behavior of 12 significant different annual biomass ash samples, and several model compounds, such as SiO2, KCl, K2SO4, K2CO3, CaSO4, CaCO3, and some of their mixtures. The STA analyses provided information on mass loss and exothermic/endothermic reactions as a function of sample temperature. The mass loss curves of the biomass ashes during STA heating could, in most cases, be explained from the chemical composition of the ash. Below 800 degreesC, weight loss is caused by the release of CO2 from CaCO3. At temperatures of 850-1150 degreesC, KCl evaporates and K2CO3 reacts with SiO2 and CO2 is released. Above 1150 degreesC and up to 1450 degreesC, a weight loss from the ash samples may be caused by the release of K2O produced from the K2CO3 decomposition or from silicates or the release from sulfates. However, even at 1450 degreesC, a large fraction of the potassium derived from carbonates remains in the condensed phase in the form of silicates.