Miscanthus x giganteus is one of the energy crops being considered to help contribute to sustainable biomass for energy production. In combustion, many biomass are characterised by low ash melting behaviour leading to slagging and fouling problems. In this study, a sub-set of samples from a larger Miscanthus agronomy trial have been assessed and studied for their ash melting behaviours using the standard ash fusion test (AFT). The samples selected are from Miscanthus grown under three different fertiliser application regimes and sampled at different times between September-March. Where feasible, leaves have been separated from stems so that the two components can be studied separately. Results are presented concerning ash content and metal analysis, and ash fusion temperatures. Some Miscanthus ash produced rapid evolution of gases during the ash fusion test resulting in swelling of the test pieces. A limited number of ash samples are also investigated further for decomposition and ash melting by simultaneous thermal analysis - mass spectrometry (STA-MS). A number of ash behaviour indices are tested, including the alkali index (a fouling indicator), the base-to-acid ratio and the base percentage, and correlations are examined between some of these and the ash melting temperatures. Miscanthus harvest time has a significant affect on the ash melting behaviour of the crop. Fusion temperatures in Miscanthus leaves are increased as the crop dries and senesces over September to March. While there is some scatter on the data, and the number of samples is small in this study, the general trend is that fusion temperature in Miscanthus stems/whole crop decreases during the period September to March. Although no single index can describe ash behaviour for all ash samples, for Miscanthus ash, it seems that wt% K2O in the ash has very good (non-linear, parabolic) correlation with melting temperature, compared with indices such as base to acid ratio and base percentage. Phosphorous content cannot be ignored in Miscanthus ash analysis, as it seems to lower the ash melting temperature and improve correlations when included in the base to acid ratio. The interpretation of melting behaviour from the STA test is more complex, although these results suggest that the endotherm minimum temperature can be a good estimate of the hemisphere temperature measured in the ash fusion test, to within approximately 25 degrees C. Further work is required to extend this study to additional biomass types. (C) 2011 Elsevier Ltd. All rights reserved.