Two samples of Polish different rank coals have been subjected to reduction and reductive alkylation in the potassium/liquid ammonia system. The samples have been studied by dilatometric (the Audibert-Arnu method), plastometric (the Giseler-Hoehne method) and thermogravimetric methods. The low-molecular-weight substances generated in the coal under the effect of potassium in liquid ammonia were found responsible for a great increase of dilatation and a shift of the softening temperature and the temperature of the maximum contraction towards lower temperatures. It was also shown that fragmentation of the coal structure led to a decrease of the temperature of the beginning of plasticity, the temperature of the maximum plasticity and to an increase of the index of maximum plasticity. The influence of C- and O-alkylation on the results of the thermal analysis was also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.