Abstract: A closure vessel assembly for use in laboratory work within a range of temperature from -196.degree. C. to 100.degree. C. above zero includes a vessel made of a plastic material and having a wall defining the vessel mouth, and a closure for closing the vessel. The closure has an annular sealing lip which depends from the top wall of the closure and extends into the vessel mouth when the closure is locked on the vessel. The sealing surface of the sealing lip engages the sealing surface of the vessel wall, in the locked position of the closure with the vessel, to provide a seal therebetween. The respective radii of the sealing surfaces of the sealing lip and the wall are selected so as to provide for minimum surface pressure yet sufficient sealing between the sealing surfaces, despite any deformation of the materials of the sealing surfaces during the required closing time of the vessel.