The degradation of organic compounds in an aqueous solution induced by a gaseous plasma, which was sustained locally on the surface of solution by means of contact glow discharge electrolysis, was mechanistically studied. The oxidation of phenol was extensively examined and it was revealed that the rate of disappearance followed the first-order rate law when the initial concentration of phenol was lower than 100 mM. As the initial concentration increased, the rate equation gradually deviated from the first-order and eventually shifted to zero-order above 250 mM. Such a kinetical behavior could be rationalized by assuming the reaction scheme where hydroxyl radical would act as the most responsible key-species for the organic degradation in competition with the coupling to hydrogen peroxide. In addition, the catalytic effects of Fe ions in the solution on the rate of organic degradation were closely investigated and could be consistently explained on the above concept.