A chemical model of kerogen decomposition is described. The model is unique in that the fate of the major carbon functionality of kerogen is considered in terms of its partitioning in the reaction products. Experimental verification of the model is based on isothermal kinetic experiments conducted as 375supO/C on a Colorado oil shale. Solid and liquid state sup15/C NMR techniques are employed to measure the distribution of aliphatic and aromatic carbon during the reaction. The results suggest that kerogen decomposition can be considered in terms of aliphatic and aromatic components of the kerogen. The aliphatic components largely determine the oil yields, whereas the aromatic components tend to remain in the residue. It is suggested that some of the residue carbon is related to a residue component already in the starting material.