Infrared studies are reported that have as their objective a better understanding of the structure of native petroleum asphaltenes as contrasted to other asphaltic substances. The aromatic clusters of the native petroleum asphaltenes appear to be primarily pericondensed in contrast to katacondensation of those in coal. Heat treatment of petroleum results in the appearance of katacondensed structure in the asphaltene. Condensed naphthenic groups are shown to be present and to vary with the source of the crude oil and its treatment prior to separation of the asphaltic fraction. The proportions of the total number of carbons existing as naphthenic, paraffinic and, specifically, as terminal methyl groups have been estimated.