Nine bituminous coals of different rank and geographical origin were carbonized at pilot scale coke oven (300 kg) in order to study the pressure generated during coking. At the same time their contraction/expansion was assessed by means of the Koppers-INCAR test. Semicokes were carefully recovered from the test so that their structure could be studied. The semicokes were separated into two parts, i.e. one that had been heated to 575 degrees C and the other that had been heated to 700 degrees C. The true and apparent density of the semicokes was measured together with their pore size distribution by means of mercury porosimetry and the results were related to the dangerousness of coals. The structure of the semicokes from safe and dangerous coals is different especially in those obtained at lower temperature. In addition, the fissures of the semicokes were evaluated. The area of the fissures was found to be greater in the case of non-dangerous coals. (C) 2008 Elsevier Ltd. All rights reserved.