The thermoplastic behaviour of 78 binary blends of Australian coking coals was measured using proton magnetic resonance thermal analysis. Their measured extents of fusion were compared with the values calculated from measurements made on the component coals assuming additivity. Significant differences between calculated and measured results were found for most blends, though only at temperatures between 400 and 520 degreesC: the coals interacted at these temperatures in a way that affected their fluidity. Both positive and negative differences were observed. The magnitude of the differences increased both with increasing differences in rank between the coals and differences in fluidity between the coals. A statistical study of the differences showed that material that became fluid in coal at temperatures below about 360 degreesC did not appear to contribute to the interactions, which suggests that fluid material derived from liptinite plays a much smaller role in interactions than fluid material derived from vitrinite or inertinite. Additionally, the study indicated that the less fusible material in a blend slightly reduced the extent to which the associated more fusible material fused. It was not acting as an inert diluent. Fifteen blends of six Argonne premium coals were examined to see if the relationships found for Australian coals between the magnitude of the interaction and coal properties could be generalised. In most cases the agreement was good. However, at some temperatures, blends of Upper Freeport coal with lower rank coals were far less fluid than expected, suggesting that the fluidity of Upper Freeport coal is especially sensitive to these low rank coals. The general effect of interactions between coals of different ranks was to narrow the thermoplastic temperature range of the blend without reducing the maximum fluidity, in effect making the thermoplastic profile of the blend resemble the profile expected from an individual coal of the same average rank as the blend. The interactions are attributed to transfer of plasticising volatile material between the coals. Crown Copyright (C) 2002 Published by Elsevier Science Ltd. All rights reserved.