The carbonisation of three coals type 35.1 in blends with coal type 35.2 has been studied in a laboratory unit using X-rays. It was found that, during carbonization of coal type 35.1, synergetic effects were observed under the action of coal type 35.2 producing an increase in the intralayer pressure, a change in the plastic layer thickness and a displacement of the non-volatile mass compared to the expected values. As a consequence of the synergetic effect, a solid residue (coke) was obtained, which had a different volume than that calculated by the additivity rule. (c) 2004 Elsevier B.V. All rights reserved.