We used three different catalysts (bulk MnO 2 , bulk TiO 2 , and CuO/Al 2 O 3 ) to oxidize phenol in supercritical water in a tubular flow reactor. CuO/Al 2 O 3 was the most active of the three on a mass of catalyst basis whereas MnO 2 was the most active on an areal basis. All three catalysts largely maintained their activities for phenol disappearance and for CO 2 formation throughout more than 100h of continuous use. MnO 2 and TiO 2 were stable in the sense that no Mn or Ti was detected in the reactor effluent. The CuO/Al 2 O 3 catalyst, on the other hand, was not stable. Both Cu and Al were detected in the reactor effluent. The bulk transition metal oxide materials experienced a 3-4-fold reduction in specific surface area after exposure to supercritical water oxidation (SCWO) conditions, whereas the supported CuO/Al 2 O 3 catalyst experienced a 20-fold reduction. Being used as an oxidation catalyst in supercritical water transformed the bulk MnO 2 into Mn 2 O 3 , the CuO catalyst into Cu 2 O, the Al 2 O 3 support into AlO(OH), and anatase TiO 2 into rutile TiO 2 . Of the three materials considered, bulk MnO 2 appears to be the best oxidation catalyst for supercritical water conditions. It is stable under reaction conditions, and it provided high activities and good activity maintenance.