The oxidation of n-heptane, I-octene, toluene, cis-decahydronaphthalene, mixtures of them, 4-phenyl-1-butene, 1,2,3,4-tetrahydronaphthalene, and three commercial diesel fuels, all in the liquid phase, by means of low pressure high-voltage oxygen plasmas was studied. Oxygen pressure was 0.2 mbar, applied power was 35 watts and reaction times ranged from I min to 23 h. Both individually and forming part of mixtures, olefins were the most reactive with ground-state atomic oxygen, 0((3)p). Olefinic double bonds reacted ca. 150 times faster than C-H bonds. Products were: epoxides and aldehydes for olefins; alcohols and ketones for alkanes; phenols for aromatics. Addition of 4.7-7.8% wt of oxygen was achieved for the diesels, depending on the particular composition, those with higher content of olefins being favoured, followed by those with higher content of alkanes. (C) 2003 Published by Elsevier Science Ltd.