Water under supercritical conditions (T > 374 degrees C, P > 221 bar) is an attractive medium for the efficient oxidation of wastes. Oxidation of carbon monoxide and hydrogen in supercritical water was studied at 550-570 degrees C and 118-263 bar in an isothermal, isobaric, tubular flow reactor. Carbonmonoxide oxidation experiments at 246 bar and 550 and 560 degrees C confirmed an oxygen dependence seen earlier and revealed an induction time of approximately 2 s in the oxidation reaction. Hydrogen formation during CO oxidation was str on the fuel equivalence ratio, with fuel-rich conditions favoring its formation. Hydrogen formation occurs very slowly in the complete absence of oxygen. In the pressure range 118-263 bar, oxidation of both hydrogen and carbon monoxide is strongly pressure (water density) dependent, with higher densities favoring higher oxidation rates. At 550 and 570 degrees C, respectively, the rates of hydrogen and carbon monoxide oxidation increased by a factor of 3-5 when the operating pressure was increased from 118 to 263 bar. Limited studies of hydrogen and carbon monoxide oxidation in a packed reactor showed that the additional surface area inhibits oxidation, although the magnitude of the effect implies that high-temperature results from the tubular reactor may be treated as homogeneous.