Utilisation of brown coal in slagging gasification technologies requires a detailed assessment of coal ash chemistry in terms of potential slagging behaviour. We investigated the phase compositions of four categories of brown coal ashes and synthetic slags after processing at 900-1100 degrees C and 1200-1600 degrees C, respectively, under reducing conditions similar to those used in coal gasification. We also determined the ash fusion temperature (AFT) of ashes and slag viscosity, and discuss these in terms of ash and slag compositions. It is found that the liquid phase appearance at low processing temperatures (900-1100 degrees C) strongly depends on ash mineral composition and primary phase field of the bulk composition. However, the ash compositions are not in phase equilibria, as well as liquid phase appears at temperatures lower than the AFTs. Slags processed at high temperatures (1200-1600 degrees C) generally follow phase equilibria, although some differences in calculated and observed compositions were identified. For practical applications in gasification technologies, ash with high silica and/or high-alumina compositions may suit entrained-flow gasifiers. Some of these ashes require fluxing or an adjustment to the silica:alumina ratio to obtain suitable slag viscosity. Coals from high-iron-magnesium ash and high-calcium ash are more suitable for use in non-slagging gasifiers. (C) 2015 Elsevier B.V. All rights reserved.