Abstract: A pipeline isolating device for istallation into and occlusion of a pressurised liquid or fluid supply pipe comprising a substantially solid and deformable boot (1) wich is adapted to move between an expanded and an unexpanded state and a shaped plunger (2) adapted to move into and out of said boot (1) wherein the boot (1) has a hollow interior (3) with a displacement contour (6) adapted to receive and cooperate with said plunger (2) so as to cause said boot (1) to move between said expanded state when the plunger (2) is inserted into said hollow interior (3) of the boot (1) and said unexpanded state when said plunger (2) is withdrawn from the interior (3) of the boot (1).