The effects of variables such as heating rate, particle size and sample mass on the extent to which a number of Australian coals and their blends fused, as measured by proton magnetic resonance thermal analysis (PMRTA), were examined in order to elucidate the thermoplastic behaviour of coking coals. The measurements were made with sample held in a Pyrex tube and it was found that the measured maximum extent of fusion of the coal was found to increase significantly with increasing depth of sample over the range 6-20 mm, with the more fusible coals showing a greater effect. Brown coals and high volatile bituminous coals substantially reduced the extent to which a bituminous coal fused to below the values expected if they were acting as inert diluents. These variations in the degree of thermoplasticity of coking coals are attributed to changes in the gas-phase concentration of volatile plasticising material in the atmosphere surrounding the coal particles. If so, the fluidity of coals can be modified by controlling the atmospheric concentration of the volatile plasticising material. (C) 2000 Elsevier Science Ltd. All rights reserved.