Decomposition of aqueous acetone was performed using a direct current (DC) plasma torch at atmospheric pressure. The torch can generate the plasma with water as the plasma-supporting gas in the absence of any additional gas supply system and cooling devices. The results indicated that 5 mol% acetone was drastically decomposed by water plasmas with energy efficiencies of 1.7 x 10(-7) mol J(-1). The major products in the effluent gas were H(2) (60-70%), CO(2) (5-16%), CO (6-16%), and CH(4) (0.2-0.9%). However, trace levels of formic acid (HCOOH) and formaldehyde (HCHO) were observed in the liquid effluent. Based on the experimental results and information from the literature, the following decomposition mechanism was proposed for acetone in water plasmas: first, electron dissociation in arc region generates acetyl (CH(3)CO) and methyl (CH(3)) radicals; then, chemical oxidation or reduction in plasma flow region forms CO and CH(x)((x:1-3)) radicals there. Finally, the generated intermediate species undergo complex reactions to form stable compounds such as CO in downstream region. However, if little oxygen is present, those intermediate species easily recombine with each other or are oxidized by OH to form unwanted by-products, such as HCOOH and HCHO. (C) 2011 Elsevier Ltd. All rights reserved.