The hydrothermal processing of municipal-waste-plastics-derived fuel oil (kerosene fraction; Cl content = 62 ppm, N content = 1150 ppm) under sub- and supercritical conditions has been investigated so as to demonstrate the possible use of water and aqueous solutions of metal salts and hydroxides for the dechlorination and denitrogenation of the fuel oil. The hydrothermal processing was carried out in a small SUS316 stainless steel batch reactor under nitrogen atmosphere. Although the two reactions took place in water, they proceeded much more readily under basic conditions, especially in aqueous solutions of alkaline metal hydroxides. That is, the nitrogen content in the product oil decreased to 297 ppm upon processing with water for 15 min at 425 degreesC, whereas it decreased to 49 ppm when 0.10 mol/L NaOH was used instead of water at 375 degreesC. Under these hydrothermal conditions, the chlorine content in the product oil was always nearly 0 ppm. Organic acids such as benzoic acid and phthalic acid in the fuel oil could also be removed.