Pressure vessel filled with at least one medium, where a gas mass enclosed by an envelope is pre-pressurized by a fluid, especially for volume compensation in piston-cylinder assemblies. The separating means is provided in an annular space, which is bounded by an interior wall and an exterior wall, and where at least one port for the annular space and at least one port for the enclosed gas mass are provided. The ports are aligned when the piston-cylinder assembly is inserted in the pressure vessel, which moves seals into their sealing positions.