Abstract: A flexible container closure, which is preferably formed of a metallic foil, is disclosed. This closure has a generally planer peripheral surface which may be sealed by either adhesive bonding or heat sealing to a container body. The closure includes an initially inwardly concave central portion which, in combination with the container walls, defines the container volume. This inwardly concave portion is capable of deformation upon increased pressure within the container to increase the volume of the container and reduce the internal pressure therein. By employing the closure structure of the present invention, the chances of leakage failure in the seal between the closure and the container body is substantially reduced.