Apparatus using cold-seal pressure vessels of molybdenum-0·5 %-titanium alloy, where each vessel is protected from oxidation by an argon gas jacket, is described. At 1000 bars pressure these vessels can be used for long runs at temperatures up to 1200° C, an increase of approximately 300° C over the range of conventional Stellite vessels. The possible use of other alloys to extend the range of this type of apparatus further is briefly discussed; pure tungsten has been shown to be too brittle.