The purpose of this paper is to investigate the decomposition mechanism of organic compounds by water plasmas. The plasma torch can generate 100%-steam by DC discharge without a commercially available steam generator. Methanol or ethanol used as a model substance of water-soluble organic compounds was mixed with water for plasma supporting gas. The main gases after the decomposition were H(2), CO, and CO(2). The 50 wt.% of carbon was transformed into solid carbon in 5 mol%-ethanol decomposition, while the solid-carbon formation from 5 mol%-methanol was negligible. Larger amount of solid-carbon formation from ethanol decomposition indicates the different mechanism between methanol and ethanol decomposition. (C) 2009 Elsevier B.V. All rights reserved.