Lumps of six bituminous coals, from 20 to 40 mm in size, were devolatilized in a laboratory oven in nitrogen atmosphere at different final temperatures ranging from 300 to 800 degrees C. The structure and morphology of the resulting chars with different degree of devolatilization have been examined under an optical microscope in order to better understand the formation mechanism of different types of char. The swelling of the caking coals and the fissuring of the non-caking coals were characterised by image analysis and some correspondences between the distribution of lithotypes within the initial coal lumps and the char structure obtained were revealed. The relation between chars structure and properties was also investigated. The char lumps obtained from caking coal exhibit better resistance to breakage than their parent coal lumps while non-caking coals show the opposite behaviour. For both caking and non-caking coals, a significant decrease of resistance is observed in the intensive devolatilization temperature range from 400 to 600 degrees C. (C) 2009 Elsevier B.V. All rights reserved.