Mineral impurities in coal form ash and a part of the fly ash form deposits in pulverized coal-firing furnaces. Understanding of the transformation of mineral matter in coals to flyash and deposit formation has improved knowledge and helped industrial engineers better handle ash-related problems. In Australia, ash fusibility tested in accordance to the standardized procedure or measured by Thermo-Mechanical Analysis (TMA) has been widely used to compare and predict slagging potential of various coals. The current study aims at obtaining an understanding of the sensitivity of TMA analysis to the physical, chemical and morphological properties of coal combustion deposits. In the study combustion residues including ash prepared in a laboratory oven at a temperature of 815 degrees C, flyash collected in a pilot scale furnace and deposits collected from a utility furnace generated from one Australian coal are used for TMA analysis. Ash samples with various levels of iron content were obtained from different milling performance, ash samples with various silicon and alumina contents were prepared by mixing ash with quartz, kaolinite and bauxite. Results indicated that TMA measurements on coal ashes are very sensitive to iron content and can be used to indicate iron related slagging problems in pf-fired boilers. For ash deposits, both the physical properties such as their homogeneous/heterogeneous nature and ash chemistry affect TMA measurement. (C) 2007 Elsevier B.V. All rights reserved.