A method and device for oxidization of materials in supercritical water. The method involves a) introducing a fluid containing water and an oxidizing agent in a ring-shaped area and through a first end of a substantially tube-shaped reactor comprising an external wall and an internal tube, b) heating the fluid in the ring-shaped area, c) introducing the heated fluid into the internal tube and simultaneously introducing material to be treated into said internal tube at a second end of the reactor, d) mixing the fluid and the material to be treated in a first portion of the internal tube, followed by cooling the obtained mixture in a second portion of the internal tube, and e) isobarically discharging the fluid/oxidized material from the internal tube of the reactor.