Abstract: In a cap assembly (1, 15) for a container, comprising an easily deformable plug member (9, 21) and a neck member (3, 16) having an opening. The neck member is adapted to cover an upper part of a container neck and partially covers the plug member (9, 21). A cover member (6, 19) is provided for the plug member (9, 21). This cover member (6, 19) is provided with a cavity (10, 22), which at least on one location increases in its cross section in the direction of opening. A locking memb4er (9', 23) is engagable positioned within the cavity (10, 22) centrally of the cap assembly. This locking member (9', 23) consists of an easily deformable material which is secured to the neck member and is cast in and so shape-matingly engages in the cavity. As a result, the locking member (9', 23) can be pulled from the cavity but cannot be reinserted into the cavity. This provides an indication whether anyone has tampered with the conainer.