Decomposition of acetone, methanol, ethanol, and glycerine by water plasmas at atmospheric pressure has been investigated using a direct current discharge. At torch powers of 910-1,050 W and organic compound concentrations of 1-10 mol%, the decomposition rate of methanol and glycerine was over 99%, while those of acetone and ethanol was 95.4-99%. The concentrations of H-2 obtained were 60-80% in the effluent gas for any compounds by pyrolysis. Based on the experimental results, the decomposition mechanism of organic compounds in water plasmas was proposed and the roles of intermediate species such as CH, CH3, and OH have been investigated; CH radical generated from organic compounds decomposition was easily oxidized to form CO; incomplete oxidation of CH3 leads to C2H2 generation as well as soot formation; and negligible amount of soot observed from glycerine decomposition even at high concentration indicated that oxidation of CHx(x:1-3) was enhanced by OH radical.