The carbonisation of two Polish coals of different caking capacity was studied in a laboratory unit using X-rays. Based on the X-ray pictures of a coal charge being heated with some markers a dynamic pattern was constructed for a coking process of well caking coals. It is shown that a displacement of the non-volatile mass of the coal charge takes place causing a change in the charge volume and forming porosity in the solid residues. On the basis of the marker trajectory during heating it is shown that on coking good caking coals the volume change of the charge proceeds stepwise. A relationship was established between a compaction of the foamy mass in the plastic layer of pod caking coals and the thickness of the gas-saturated zone in it. The X-ray data have been confirmed by experimental data using scanning electron microscopy. (C) 2004 Elsevier Ltd. All rights reserved.