A device for treating pieces of a substrate at high pressure, piece by piece or in batches, with a treatment medium in the supercritical or near-critical state, includes a first pressure chamber, a pipe system for supplying and discharging the treatment medium, to and from the pressure chamber under high pressure and an electric motor, which is fixed to the housing in the first pressure chamber or in a second pressure chamber coupled to the first pressure chamber under the high pressure, for driving an actuator to carry out a mechanical action in the treatment medium. The electric motor is in this case open to the treatment medium and is disposed and designed to be accessible to the treatment medium, in such a manner that during the treatment of a substrate the treatment medium flows through and around the electric motor.