The E.S.R. g-values of a variety of bituminous materials have been plotted against a function of their heteroatom content ∑ δκXκ, where δ is the splitting coefficient of the given heteroatom κ, and X is its atomic fraction. Three discrete series are evident: two coal series, one exhibiting direct variation of g with ∑ δκXκ (Series I), the other exhibiting an inverse relationship (Series II). The petroleum asphaltenes, as well as most asphaltites and asphaltoids (Series III), lie above and roughly parallel to Series I. Sulfur-treated asphaltics approach Series I, whereas heated asphaltenes and resins are situated around the intersection of Series II and III. Bituminous deposits may transform through diagenesis to their mature forms, as evidenced by their increase in the delocalization (increased aromaticity) with a decrease in heteroatom content. The diagenesis of coal is reliant on oxygen whose elimination may result in aromatization (Series I), and further transformations may increase active peripheral oxygen function groups (Series II). The intersection of Series I and II contains the delocalization states. There exist major differences in the precursors and mode of transformation between members of the coal and asphaltene series.