1,4-Dioxane, a persistent organic pollutant, was easily decomposed to methane and CO2 under supercritical water gasification (SCWG) conditions (400 degrees C, 26 MPa) in the presence of a Ni or Ru catalyst. Charcoal-supported Ru catalyst achieved 100% decomposition of 0.01.M dioxane at 450 degrees C within 90 min. This catalyst was able to clear the current wastewater discharge limit of 0.5 mg L-1 set by the Japanese Ministry of the Environment within 60 min reaction time at 400 degrees C for the same dioxane concentration and maintained its activity up to nine times of repeated usage. It is emphasized that the present process employs no chemical oxidants such as ozone or hydrogen peroxide.