To obtain compositional information on both hydrocarbon and nonhydrocarbon components of crude oil and to develop suitable analytical methods applicable to any crude oil, the Bureau of Mines, through American Petroleum Institute Research Project 60 (api rp 60), is studying in some detail the higher boiling fractions of five crude oils of different chemical composition and geological origin. To effectively process and use the heavy ends of petroleum it is necessary to develop suitable separation and characterization procedures not currently available. A systematic procedure developed by the Bureau of Mines uses techniques such as isothermal and molecular distillation, chemical treatment, silica-alumina gel chromatography, gel permeation chromatography and spectrometry. Using these techniques, the Bureau reports herein a detailed separation and characterization of 370 deg to 535 deg c distillate from Gach Saran, Iran, crude oil. Such a systematic procedure, applicable to any crude oil, should be of value in selecting feedstocks and optimizing petroleum processing operations.