A controlled release and self-resealing vessel assembly for high pressure, microwave assisted chemistry is disclosed. The vessel assembly includes a cylindrical reaction vessel formed of a material that is substantially transparent to microwave radiation. The vessel has angular walls, one closed end for retaining reactants in the lower portions of the vessel, and one open end that defines a mouth for the vessel. The mouth of the cylinder vessel has a lip that is beveled inwardly from the open end. A microwave-transparent solid floating plug closes the mouth of the reaction vessel; with the plug have a frustum portion for engaging the beveled lip when the plug is placed in the mouth of the vessel. A lid exerts a defined axial force against the plug for sealing the plug in the open end of the cylinder with the frustum engaging the lip.