The poor repeatability and reproducibility of fusibility temperatures of the ashes with high (SiO2 + Al2O3) levels are well known. A study was, therefore, made of such ashes using thermo-mechanical analysis (TMA) to indicate the shrinkage of samples progressively heated at the same rate as the test for standard ash fusibility temperatures (AFT). The results confirm that the extent and rate of change of shrinkage of the samples with low K2O levels (< 1 wt.%) is low in regions of the deformation temperature (DT), resulting in poor accuracy of its determination. It was found that DT is mainly related to substantial melting of minerals containing K2O (i.e. illite) in these ashes. The scanning electron microscope (SEM) analysis of quenched ash samples and thermodynamic calculations were used to illustrate the effect of K2O in terms of the extent of slag generation necessary for shrinkage. The TMA test is shown to provide a more sensitive alternative procedure for characterizing the ash fusibility.