The decomposition of phenol and 4-chlorophenol by underwater pulsed discharge has been examined. The addition of hydrogen peroxide improved the decomposition rate of phenol by about 2.5 times and that of 4-chlorophenol by about 1.5 times. Analysis using a fluorescent probe suggested that the addition of hydrogen peroxide enhanced the production of hydroxyl radicals by about 3 times. We have also examined the effect of using microbubbles on the decomposition of phenol. The use of only microbubbles resulted in a slight increase of the decomposition rate, while the use of both microbubbles and hydrogen peroxide led to a remarkable enhancement of the decomposition rate by about 3.5 times with the conversion of phenol reaching 70% at discharge time of 3 h, which corresponded to the energy yield of 4.39 × 10-9 mol J-1.