The influence of density and heating rate on the processes occurring during carbonization of low rank and high rank coals was investigated using X-ray technique. X-ray pictures have shown that an increase in charge density has no or little effect on decreasing its volume during carbonization. A rapid heating results in increasing the volume of solid residues obtained after carbonization. The results of X-ray analyses have shown that the increase in charge density tends to increase the quantity of the ordered phase largely due to growth of the crystallites present. During rapid heating processes of crystallite growth also take place, however, the main contribution to the increase in the ordered phase is made by the processes of occurrence and growth of new crystallites. (C) 2004 Elsevier Ltd. All rights reserved.