This study investigated the deposit structure in terms of the bulk porosity for Australian bituminous coals and blends that were fired in a pilot-scale test furnace. The first part of this study estimated all the deposits bulk porosity using two indirect techniques: a density measurement technique and by scanning electron microscopy (SEM). The results showed a significant difference between the two techniques. The density measurement underestimated the bulk porosity of the deposit due to the closed pores within the deposit structure. The second part of this study proposed a novel experimental approach to measure the changes in the deposit bulk porosity at elevated temperatures using thermomechanical analysis technique (TMA). The TMA bulk porosity measurements were comparable with the SEM results.