A hot isostatic press comprising a working vessel, formed of at least two pieces, having a working chamber within it. The working medium supplied to the working chamber, for transmitting pressure to work pieces within the chamber, may be a molten metal. Heating means within the vessel and cooling means at the exterior of the vessel cooperate to produce a continuous isothermal region completely surrounding the working chamber. The isothermal region is at the solidification temperature of the metal working medium, so that any potential leaks of molten metal working medium from the vessel are sealed by solidification of metal reaching the isothermal region. There is a heat-resistant liner within the vessel, the isothermal region being within the liner. A storage vessel for the working medium and a high pressure vessel, which may be a plunger pump, are connected to each other and to the working vessel through conduits furnished with valves.