Decomposition of phenol at a concentration as high as 2 wt.% was effected by supercritical water oxidation at 25 MPa. Reaction temperatures ranged from 623-723 K and residence times were varied from 6.5 to 26 s. Oxygen was added in an equivalent amount to investigate reaction intermediates. The degree of phenol decomposition and reaction product were measured. Although tarry material production was observed, phenol decomposition conversion was predicted well by the reaction rate equations developed by previous researchers who conducted experiments at lower concentrations. Difference from low concentration phenol oxidation was found in the reaction product distribution and tarry material production. One possible explanation for this result is that the initiation of phenol decomposition is the same regardless of phenol concentration but that the succeeding radical reactions are different. The additive reaction between aromatic compounds was enhanced by high phenol concentration.