Abstract: The invention provides a system for the detection of leaks in the primary barrier (150), the secondary barrier (130), and the inner hull (110) of the primary and secondary insulation spaces (140 and 120) of a cryogenic cargo tank (10) carrying liquified natural gas in a transport ship (20) wherein an inert gas is maintained at a first predetermined pressure (P.sub.NP) in the primary insulation space (140) which is less than the pressure of the cargo gas (P.sub.CH.sbsb.2) and wherein the inert gas is maintained at a second predetermined pressure (P.sub.NS) in the secondary insulation space (120) so that the second predetermined pressure (P.sub.S) is less than the first predetermined pressure (P.sub.NP) and is also less than atmospheric pressure (P.sub.O). A gas sensor (930) is placed in the primary insulation space (140) to detect any leaks in the primary barrier (150) and a sensor (940) is placed in the secondary insulation space (120) to detect the presence of any oxygen due to a leak in the hull (110).