There is disclosed a fuel tank valve assembly 24 for a fuel tank 4. The fuel tank valve assembly comprises a discharge valve 30 which in normal operation is closed, and which can be opened to discharge fluid from the fuel tank; and a water passageway 56 across which extends a membrane 60 which is permeable to water but impermeable to fuel. There is also disclosed a fuel tank 4 comprising a fuel tank valve assembly 24.