Alkali species in coal play an important role in mineral matter reactions during gasification. Brown coal ashes can be high in sodium (up to 20 wt%), which interacts with other minerals to significantly affect the formation of slag (liquid phase). At high temperatures, sodium in slag affects phase equilibria, liquidus temperature, and slag viscosity. We investigated phase transformations in brown coal ashes at 800-1000 degrees C and slags at 1200-1600 degrees C to study the behaviour of sodium species in the range of 3-15 wt% using X-ray powder diffraction, thermogravimetric analysis and electron probe microanalysis. Sodium species appeared in coal ashes in different forms, such as sulfates, carbonates, and alumina-silicates, resulting in different ash bulk compositions. After processing under gasification conditions, alkaline species react with other minerals and may result in the appearance of a liquid phase. In this study, the liquid-phase content of ashes generally increased with increasing sodium concentration. In slags, sodium often decreased the liquidus temperature and solids content below liquidus temperature. It results lower slag viscosity in sodium enriched slags. These findings demonstrate the importance of phase transformations in brown coal mineral matter during gasification, and the potential they have to strongly affect the required operational conditions.