A tube closure device (50) is disclosed for screw-threaded engagement with the threaded end (14) of a tube (12) containing an adhesive or other chemical substance, the tube (12) being in the form of a relatively rigid cylinder of the kind employed in an adhesive gun or applicator. The tube closure device (50) is adapted to seal the threaded end (14) of the tube (12) after use to substantially prevent the ingress of air and hence preserve the contents of the tube (12) for later reuse. The tube closure device (50) is preferably in the form of a cap provided with a sealing means (56) having an internal surface (58) adapted to protrude into an open end of the tube so as to engage with the contents of the tube (12) and form an air-tight seal.