The objective of this study was to characterize internal structure of semifusinite and fusinite in inertinite concentrates (I content 80-81%) before and after heat-treatment (400-1200 degrees C), under inert conditions. Maceral concentrates were prepared using gravity separation from two samples of steam coals (R(r) = 0.98 and 0.99%) and two samples of coking coals (R(r)- 1.01 and 1.42%) collected from the same seam. Micro-FTIR and reflectance (R(r)) measurements were carried out. Mass loss of the concentrates upon heating was also determined. Semifusinite in concentrates before heating is characterized by lower aromaticity and condensation of the structure (lower values of CH(ar)/CH(2) + CH(3) and CH(ar)/C=C(ar) ratios) than fusinite. The difference in aromaticity between semifusinite and fusinite is similar despite the rank of the parent coal. Semifusinite from the coking coal is characterized by lower C=O/C=C(ar) ratio as well as higher aromaticity than semifusinite in the steam coal. Statistical analysis showed occurrence of two types of semifusinite which differed in aromaticity and condensation of the structure. Fusinite from the higher rank coal has higher aromaticity in comparison to fusinite from the lower rank coal. Under heating, semifusinite of both types and fusinite experience an increase in aromaticity and condensation of the structure, which results in significant reflectance increase. Strong correlations between the values of both CH(ar)/CH(2) + CH(3) and CH(ar)/C=C(ar) ratio and reflectance of semifusinite and fusinite in concentrates before and after heating have been found. The most significant alteration of the chemical structure of semifusinite occurs at 600 degrees C and 800 degrees C (coking coal) as well as 800 degrees C and 1000 degrees C (steam coal) and for fusinite at 600 degrees C and 800 degrees C. After heating at 1200 degrees C, semifusinite in the steam coal concentrates has lower C=O/C=C(ar) ratio and much higher aromaticity than semifusinite in the coking coal concentrates. Fusinite in the coking coals after heating the concentrates at 1200 degrees C has lower C=O/C-C(ar) ratio and condensation of the structure but higher aromaticity than semifusinite. Rate of aromaticity increase in 400-1200 degrees C range calculated for semifusinite is inversely proportional to aromaticity of this maceral in concentrate before heating. (C) 2010 Elsevier B.V. All rights reserved.