Initiation of self-heating is governed by a coats ability to oxidize at low temperatures. Aliphatics undergo low temperature oxidation and are the main constituents of vitrinite. Conversely, the chief constituents of inertinite, aromatics, oxidize at elevated temperatures. In this study, published elemental data for the Witbank Coalfield are used to calculate the H/C ratio and aromatic fraction (fa). The parameters are correlated with petrographic composition. An increased proportion of aliphatics in vitrinite suggests that it has higher heat generating capacity (HG) at low temperatures than inertinite of similar rank. When the heat retention capacity,(ER) exceeds HG, self-heating initiates, resulting in oxidation of aromatics. Sustained self heating, initiated by oxidation of vitrinite macerals, may trigger ignition. Spontaneous combustion in vitrinite-poor coals is indicative of a coal body with high HR For Witbank coals, the likelihood of self-heating is classed as high- and low-risk corresponding to vitrinite- and inertinite-rich coals, respectively. By implication, spontaneous combustion tests quantify HG.