This study investigated the plasma reforming process for diesel focusing on the relative ratio of oxygen to fuel. Excess O(2) in the partial oxidation process is known to increase the combustion portion, resulting in a decreased yield of H(2) and CO. However, in this parametric investigation, there was no apparent decrease in the H(2) and CO selectivity. Adding O(2) did not increase the portion of combustion in the overall reaction. Rather, an excess O(2) supply from partial oxidation stoichiometry resulted in an increase in CO(2) selectivity without a reduction in CO selectivity. Heavy hydrocarbon species were identified as a source of CO(2) in excess O(2) conditions due to preferential oxidative cracking. The additional oxidation of C(1)-C(4) ,species by excess O(2) provided a minor contribution to CO(2). Excess O(2) affects soot generation characteristics by suppressing the agglomeration of soot particles, resulting in smaller particle generation. However, the oxidation of soot particles does not provide a major contribution to increasing the CO(2) selectivity. The results show that in a real reforming process, controlling the O(2) supply does not have a strong effect on the process selectivity of hydrogen. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.