Oxidation of several liquid, long chain hydrocarbons (alkanes, alkenes, arenes, and a light gas oil, LCGO, product of a coking process) has been studied by using a low-pressure high-voltage oxygen plasma. Olefins were readily oxidized to epoxides and carbonyl compounds, O(P-3) being the active species in the plasma. At long treatment times the saturated chains of the olefins also reacted to produce secondary alcohols and ketones. These compounds and tertiary alcohols, only when tertiary carbons were present, were obtained with alkanes. At 5 min 4-phenyl-1-butene showed reaction mostly on the olefinic C=C, followed by oxidation of the aromatic ring. At 120 min the olefinic fraction oft;he compound was eliminated and the reaction of the saturated -CH2-groups represented only 0.6% of the total conversion. LCGO, with a composition of 65.1% alkanes, 34.8% aromatics, and 3.1% olefins, was treated under similar conditions used for the model compounds. At 120 min the olefinic fraction of the mixture was eliminated and the cetane number, a parameter indicating the quality of fuel oils, was improved by 66% without any additional treatment.