In situ low-temperature air oxidation studies of subbituminous coal have been performed at 77, 125, 200, 300, and 400°C by diffuse reflectance Fourier transform (DRIFT) spectroscopy. The oxidation reaction proceeds via oxygen insertion at aliphatic sites in the coal structure, which progressively produces aldo/keto groups, acid groups, and acid anhydride entities with the simultaneous consumption of hydrogen at these sites. The production of anhydrides occurs even at the lowest temperature (77Ã‚Â°C), but only above 200°C is there sufficient mobility of the acid functionalities for major quantities of the anhydride species to be formed. Above 400C, the anhydro groups predominate in the steady-state production of carbon dioxide and water vapor. In addition to the detailed information concerning the carbonyl species, the spectra of the oxidized coal reveal some new information regarding the aromatic C-H stretching bands, which can be studied in some detail unencumbered by interference from the aliphatic bands that have been removed in the oxidation process. Further details related to the aromatic bands are revealed by deuterium exchange of the remaining 0-H groups (primarily phenolic type 0-H) in the oxidized coal structure. This exchange removes these bands from overlap with the broad 0-H stretching band resulting from hydrogen bonding of the 0-H groups. The present study reveals further merits of in situ DRIFT analysis in extending the knowledge of coal structure and reactions. The study also indicates much potential for further work.