Sintering tests at temperatures between 550°C and 950°C were carried out on wood-based biofuels. The aim of the sintering experiments was to characterize the temperature-dependent mineral reactions that lead to the formation of critical and uncritical slags in combustion chambers in laboratory scale and to compare it with the composition of real slags from BMHP plants. The properties of slags, which are formed during combustion are defined by their chemical and mineralogical composi-tions. During heating of samples, temperature-dependent mineral reactions take place, which are asso-ciated with degradation of existing mineral phases as well as formation of new mineral phases. Results so far show that mineral reactions in ash are mainly due to the content of calcite (CaCO3), various lay-ered silicates and quartz (SiO2). As the temperature rises, there is an increased formation of calcium-containing silicates, which can also contain agnesium, iron and less pronounced aluminum, whereby these mineral phases also occur in real slags. In the future, the sintering tests should offer information about possibilities to reducee formation of critical slags by using biomasses or biomass mixtures with an optimized chemical composition, which can lead to the formation of slags with an uncritical mineralogi-cal composition.