The inertinite combustion reactivity of the Herrin #6 and the Murphysboro coal seams were examined. Three samples of the Herrin #6 seam were analyzed: A whole coal sample, a resonance disintegration milled sample, and a fusain lithotype sample. A fusain lithotype sample was also analyzed for the Murphysboro coal seam. Density gradient centrifugation (DGC) techniques were used to separate the inertinite components of each sample into 26 discrete density fractions. Fractions were chosen based upon changes in the density profiles. The thermal properties of the fractions were analyzed by thermogravimetric (TGA) analysis. The results showed that although the reactivity profiles for the coal samples were different, discontinuities occurred in all of the coal samples across the same density intervals. It also illustrated that the change between semifusinite and fusinite was not a continuous change as suggested by the definition (ICCP, 2001, but was instead shown to be discontinuous. Changes within the chemical structure were inferred from these discontinuities indicating differences within the semifusinite and fusinite maceral groups. The boundary between more/less reactive macerals was shown to increase with density as temperature increased indicating that at higher energies more macerals were reactive during the combustion process.