Among the many complicated chemical and physical changes occurring in coal in the plastic temperature range, the authors focused on the changes in the size of polycondensed aromatic rings since the aromatic moieties are believed to play a role in governing the plastic phase. Three Argonne premium coal samples, Upper Freeport, Pittsburgh No. 8 and Illinois No. 6 coals, were employed in the present study. The average size of the aromatic rings for coal and heat-treated coal (char) was evaluated by solid-state NMR measurements; the results strongly indicated that the development tendency of the aromatic ring size for the semi-coke samples was different among the three coals. A mechanistic view of the plastic phenomena of coals is discussed with the assistance of the weight loss during heat treatment, and the amount of transferable hydrogen in coal or semi-coke samples. (C) 2004 Elsevier B.V. All rights reserved.