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Decomposition of environmental contaminants such as phenol contained in water was investigated using a pulsed high-voltage gas-phase discharge on the water surface (water surface plasma). The discharge consists of streamer channels that spread out over the water surface. Discharge characteristics were dependent upon the distance between the needle-tip electrode and the water surface, the shape of the submerged ground electrode, and the composition of the gas enveloping the electrode. When the electrode-water distance was decreased, the discharge mode changed from corona to streamer, and then, finally, to a water surface discharge when the distance was small. Argon gas was the most effective enveloping gas for decomposing phenol in water (compared to oxygen or air). When the gas flow rate was increased to carry away the active species formed in the gas phase; the decomposition rate did not change in argon, but decreased in oxygen. The shape of the submerged ground electrode influenced the discharge state and the phenol decomposition rate. A ring-shaped ground electrode was more effective for decomposition of phenol than straight or semicircular shapes. Experiments were performed to identify the mechanism(s) responsible for the decomposition of organic materials in water.