The distribution of hydrogen according to structural type in a series of representative asphaltic materials was investigated by proton nuclear magnetic resonance (NMR). Determination of the fraction of total hydrogens, which are saturated, hs, by using the NMR technique was shown to be more accurate than by infrared spectrophotometry. Also, by utilizing data on the diameter of the aromatic sheets, La, obtained by X-ray diffraction, the shape of the aromatic centers was shown to vary but, on the whole, to be highly condensed and in no case chainlike in nature. Approximately 48 to 70 percent of the positions on the edge of the aromatic centers are substituted by atoms other than hydrogen. While the presence of naphthenic groups was confirmed, lack of reference data did not permit determination of size, shape, or degree of substitution. For the fractions prepared from crude oils, approximately 29 to 66 percent of the terminal carbons are in methyl or ethyl groups attached to the aromatic centers. NMR results confirmed the conclusion reached previously that mild thermal cracking results in the cleaving of chainsand dehydrogenation of the naphthenic structures to aromatic structures.