The fusion of ashes produced during solid fuel combustion greatly affects the tendency of these ashes to cause operational problems in utility boilers. In this paper, a new and quantitative laboratory method for assessing the fusion of ashes based on simultaneous thermal analysis, STA, is described. Using STA, melting is detected as an endothermic reaction involving no change in mass. The measurement signals are transferred into a fusion curve showing the melt fraction in the ash as a function of temperature. This is done either by a simple comparison of the energies used for melting in different temperature ranges or by accounting for the relevant melting enthalpies. The method repeatability is good, melting onset determinations and completions generally within 10 degrees C, and melt fractions at given temperatures generally within 10% melt. Results are presented for simple binary salt mixtures, for which the agreement with fusion as determined by phase diagrams is very good, and for straw (salt-rich) and coal (silicate-rich) ashes. Comparing ash fusion curves to index points of current standard ash fusion tests showed initial melting at temperatures typically between 50 degrees and 100 degrees C - but in extreme cases as low as 260 degrees C - below the melting onset as found by the standard fusion tests. Characterizing the fusion by STA provides a more detailed description of the ash fusion as compared to conventional methods, and the onset of ash fusion is more precisely determined. Furthermore, in combination with, e.g. computer-controlled scanning electron microscopy, the method enables identification of the chemical species melting in different temperature ranges. Since ash melting has a major impact on the deposit formation tendency, the presented detailed ash fusion determination improves the prediction of problems related to ash deposition in boilers. (C) 1999 Published by Elsevier Science B,V. All rights reserved.