Air-release values of mineral base oils are adversely affected by certain minor constituents which are preferentially adsorbed over the dispersed air bubbles. Such components of a few selected base oils having high air-release values were isolated by adsorbent treatment and solvent-extraction steps. Among the various adsorbents and solvents tried, silica gel and isopropanol were found to be the most effective in improving air-release values of the base oils studied. The components of base oils so separated were further fractionated and analyzed. The results indicated that the fraction of the base oils responsible for poor air release is rich in aromatics, naphthenes, nitrogen, and sulfur. A theoretical method for calculating the air-release value of base oils within the framework of IP 313 experiment has been proposed. The calculated values are valid only for ideal base oils, ie, those which are essentially free from components which are adsorbed on dispersed air bubble surface. Thus, with silica-gel-trea...