In this study, deposition experiments with well characterized samples of Australian black coals and blends were conducted in a laminar drop-tube furnace to assess the blends behavior and their potential to form ash deposits. Comparison of the results between the blends and single coals shows that the blends behavior was not additive in nature. Some blends developed a thicker ash deposit layer with a maximum thickness higher than 700 mum compared to a thickness of 300 mum for the source coals, whereas, other blends had a lower potential to form ash deposits. This non-additive behavior of coal blends results from the interaction between ash particles within the deposit layer. Therefore, the coal blends performance may not be the same as that of the source coals with same bulk composition. Three techniques were used to explain the trends of the deposition experiments. The use of slag viscosity measurements and a slagging index based on the ash bulk properties, were not successful. Thermomechanical analysis (TMA) test was conducted as another technique that represents the thermal behavior of the ash deposits. The (TMA) penetration curves showed a reasonable matching trend with the deposit thickness obtained in the deposition experiments. (C) 2004 Elsevier Ltd. All rights reserved.