Several petroleum-derived materials have been structurally characterized in terms of their hypothetical averae moleule using a combination of 1H and 13C n.m.r. spectroscopy. Steps were taken to ensure that reliable 13C n.m.r. data were obtained, including adding a relaxation agent, chromium acetonylacetonate; using a pulse delay of two seconds; and running the spectra in a gated prton decoupled mode. The method involved the calculation of average molecular parameters for the petroleum fractions from 1H and 13C n.m.r, average molecular weight, and elemental analysis. This was most efficiently carried out with the aid of a computer. The structures of the petroleum residues were compared by constructing hypothetical average molecules based on these average parameters. Some significant structural differences were seen for the petroleum materials studied.