A novel direct current (DC) plasma torch, operating with a gas mixture consisting of carbon dioxide and hydrocarbon (methane), has been adapted and used for the first time for treatment of a solution containing dissolved carboxylic acid. Compared with conventional DC thermal plasma torches, this novel torch has high plasma enthalpy, high thermal conductivity, low cost and often readily available operating gas (CO2) and long electrode life-time. Performance of this CO2/CH4 DC thermal plasma torch submerged in aqueous solution was investigated for decomposition of dissolved organic contaminants. Sebacic acid was selected as a representative of organic pollutants in wastewater of chemical process industries. Effect of different operational conditions including treatment time, initial solution pH, and the reactor pressure as well as the role of oxidizing agents such as (H2O2) were investigated on decomposition rate of sebacic acid and its intermediate products. It was shown that the submerged CO2/CH4 DC thermal plasma could decompose carboxylic acid. The highest decomposition rate was obtained at neutral solution pH. Adding H2O2 could significantly enhance the decomposition rate due to synergetic effect of ultraviolet radiation (UV) absorption and hydrogen peroxide oxidation. Photo-oxidation was also proposed as the decomposition mechanism with the novel CO2/CH4 plasma torch based on its high level of UV. This work shows the ability of this novel plasma torch for treatment of organic acids in liquids and solutions and therefore offers a new application for the CO2/CH4 thermal plasma in wastewater treatment.