RE-REFINED LUBRICATING BASE OILS are base oils derived from reprocessing of used lubricating oils to remove contaminants, oxidized products, and additives. Early studies suggested that the basic molecular structures of the hydrocarbons were not altered in use, and there were no systematic variations in either contaminants or compositions in the used oils from geographical or seasonal variations *. Various processing technologies are available to remove the contaminants from used oil, most notably the acid/clay process. The acid/clay process has long been used throughout the world and was the dominant process in 1976 when National Bureau of Standards (NBS) initiated its Recycled Oil Program. Due to the environmental problems associated with the disposal of acid sludge, the acid/clay process is rapidly being displaced by new ones: e.g. hydrogenation, vacuum distillation, shortpath distillation, solvent extraction. Host new processes have some pretreatment steps to remove sludge, water, and additive metals. Re-refined base oils are primarily in the 200–400 neutral range. The light ends in the original base oils are lost through evaporation during the use cycle. The very heavy ends are usually removed during re-refining.Under the Energy Policy and Conservation Act (1975) the National Bureau of Standards is required to study re-refined lubricating base oils and develop test procedures to assess the substantial equivalency between re-refined and virgin oil products. In order to help fulfill this responsibility, a detailed study was made of the comparison between the re-refined and virgin base oil. Seventeen lubricating base oils were selected for detailed characterization in these studies, ten re-refined and seven virgin base oils. These base oils represent a cross-section of available processing technologies, crude sources, and viscosity grades. This paper describes the detailed characterization of these seventeen oils and discusses the various aspects of quality and consistency associated with re-refined base oils. Based on these results, a tentative set of test procedures is recommended for monitoring the quality and consistency of re-refined base oils for automotive applications.