Abstract: The drop-in containment sleeve is an apparatus designed to work in conjunction with a flow control device for preventing a dense liquid (such as diesel fuel) from being dispensed into a storage tank that contains a less dense liquid, such as gasoline fuel, (and vice versa) by allowing the device to seal against an inside lip of the sleeve, thereby fully- or partially-containing the fluid that activated the flow control device. In petroleum fuel applications, the containment sleeve can replace the inlet collar of the filler tube.