The reaction between naphthalene and oxygen in supercritical water (SCW; 374degreesC, 22.1 MPa) has been studied in a batch reactor of 6-ml volume placed in a fluidized bath. Reaction temperature varied from 300degreesC to 420degreesC, while pressure varied from 18 to 29.3 MPa. These conditions corresponded to both subcritical and supercritical water. Conversion of naphthalene in supercritical water reached about 99%; more than 80% of carbon appeared as CO2 within 300 s reaction time. The effect of time and O-2 concentration on CO2 yield was determined. Intermediate species formed during naphthalene oxidation in SCW were identified and quantified for the first time. They included acetic acid, ketone, and carboxyl compounds. Two main products observed in the aqueous phase were acetic acid and formic acid. Naphthoquinone, salicylaldehyde, 2'-hydroxyacetophenone, salicylic acid, phthalic anhydride, phthalide, chromone, 1,4-naphthoquinone, 1-naphthol, dibenzofuran, and fluorenone were identified during the SCW oxidation of naphthalene at 343degreesC and 212 MPa. Type and amount of intermediates varied with temperature and pressure. The following sequence of reactions was proposed for SCW oxidation of naphthalene: (1) breaking off the benzene ring; (2) formation of light hydrocarbons such as formic acid, acetic acid, aromatic carboxylic acid, ketone, and ester with one benzene ring; (3) polymerization of selected intermediates forming dibenzofuran, fluorenone, and 2'-hydroxyacetophenone; and (4) oxidation of intermediates to CO2 and H2O. Possible reaction of naphthalene oxidation via free radicals is also discussed.