An apparatus (1) for use with a container (100) for storing a substance. Typically, the substance is a volatile liquid, such as hydrocarbon fuels, alcohol or distilled liquids. The apparatus (1) acts to prevent filling and evaporative losses of the liquid, minimise internal corrosion of the container (100) and prevent humidification of the liquid. The apparatus (1) employs a bladder (3a, 3b) inside the container (100). The bladder (3a, 3b) is filled with air. A pressure control unit (7) is provided to control the supply of air to the bladder (3a, 3b) such that the pressure in the bladder (3a, 3b) is maintained at or above a selected level, being the vapour pressure inside the container (100). The bladder (3a, 3b) is maintained in an inflated condition such that it fills the space above the substance in the container (100).