Oxidation of eight liquid cycloalkanes and two diesel fuels has been studied by using a low pressure high voltage oxygen plasma. Naphthenes were individually oxidized to alcohols and ketones, O(P-3) being the active species in the plasma. Tertiary C-H bonds were the most reactive. At long treatment times the main products suffered successive oxidation to produce two oxygenated groups in the same molecule. cis-Decaline was four times more reactive than trans-decaline. For this two isomers and bicyclohexyl the conversions were the lowest, the yield of the tertiary carbon being between 40 and 80% of the total. Methyl-, ethyl-, and n-propylcyclohexane gave similar results. The reactivity of the two tertiary carbons of iso-propylcyclohexane amounted to c.a. 80%. This compound was the most reactive. Due to steric effects of the alkyl group, tert-butylcyclohexane showed low reactivity. Addition of 4.7-7.0 wt % of oxygen to the diesel fuels has been achieved in one-step process with the plasma.