This study describes changes of surface roughness of semifusinite and fusinite as an indicator of structural alteration resulting from heat treatment at 400-1200 degrees C. Surface roughness has been investigated by atomic force microscopy of inertinite concentrates from coking coals (vitrinite reflectance R(r) = 1.07%-1.41%) from the Upper Silesian Coal Basin of Poland (Namurian C - Westphalian A). Unheated fusinite has a higher surface roughness than semifusinite from the same coal. The average surface roughness of semifusinite decreases with the Swelling Index of the parent coal. Heating increases the surface roughness of semifusinite and fusinite. Increase in the average surface roughness is stronger for semifusinite than fusinite and correlates to increasing reflectance of these macerals. The surface roughness of semifusinite correlates to the relative mass loss of the inertinite concentrates during heating. After heating to 1200 degrees C fusinite has a lower average surface roughness than semifusinite from the same coal. Consequently, average surface roughness can be used as a measure of structural alteration of inertinite group macerals during heat treatment. (C) 2011 Elsevier B.V. All rights reserved.