The light gas oil fraction, C13 to C17, of petroleum was separated into an aromatic portion and a paraffin-cycloparaffin portion. Four compounds, biphenyl at 255 C, 2,6-dimethylnaphthalene near 268 C, a trimethylnaphthalene near 288 C, and a dinuclear aromatic, C14H16, near 312 C, were isolated from the aromatic portion by regular and azeotropic distillation. The paraffin-cycloparaffin portion was separated by treatment with urea to give a concentrate of normal paraffins and a concentrate of branched paraffins plus cycloparaffins. Five normal paraffins, C13 to C17, were isolated from the normal paraffin concentrate by distillation. Distillation of the branched paraffin-cycloparaffin portion yielded portions of distillate in which certain groups of compounds, for example, the monomethylalkanes, were concentrated. The C18 to C25 fraction of petroleum, heavy gas oil and light lubricating oil, was separated into an aromatic portion and a paraffin-cycloparaffin portion by adsorption with silica gel. The aromatic portion was further separated by adsorption with alumina to give portions containing mononuclear, dinuclear, and trinuclear aromatics, respectively. The paraffin-cycloparaffin portion was further separated by treatment with urea to give a concentrate of normal paraffins and a concentrate of branched paraffins plus cycloparaffins. The portion containing the normal paraffins was fractionated by distillation at very low pressure, in a 50-stage apparatus, to give concentrates of the individual normal paraffin hydrocarbons. These were separately subjected to crystallization to yield purified samples of the individual normal paraffins, C18 to C24. A description of the 50-stage apparatus for distillation at very low pressures is included. The C26 to C38 fraction of petroleum was separated into four broad portions: a “water-white” oil portion, an “extract” oil portion, a “wax” portion, and a residue portion. The water-white oil portion was separated by distillation at very low pressures in a simple apparatus and then by solvent extraction with reflux in tall columns to give several series of “homogeneous” fractions. The extract oil portion was separated in a similar manner. Certain key fractions were carefully hydrogenated to convert the aromatic rings to cycloparaffin rings. The following properties were determined on all the important final fractions: molecular weight, content of carbon and hydrogen, content of sulfur, nitrogen, and oxygen, boiling point at a pressure of 1 mm of mercury, density, refractive index, refractive dispersion, kinematic viscosity at 100 and 210 F, aniline point, and optical rotation. Mass, infrared, and ultraviolet spectral data were obtained on certain selected fractions by a number of laboratories in the petroleum industry. The results were correlated to give information concerning the type and amounts of hydrocarbons in the lubricating oil portion of petroleum.