Abstract: An improved closure device for a laboratory scale chemical reactor which provides fast and efficient opening and closing of the reaction vessel as well as providing a simple and reliable system for relieving excessive pressure should it build up in the reaction vessel. The reactor is sealed by use of a substantially flat head that is seated against the rim of the reaction vessel by pressure which is generated by a variably adjustable closure means. Since the closure pressure can be adjusted to any value within the design limitations of the closure means, the closure means also operates as a pressure relief valve.