Abstract: A security system for preventing theft of the liquid contents of storage tanks, such as those used for storing crude oil. The security system includes a first selectively locked access way, a second locked access way and a discharge valve security control subassembly accessible from outside the tank only by way of both access ways when both are unlocked. The discharge valve control subassembly is operatively associated with a housed, inaccessible discharge valve which can be opened only when the discharge valve security control subassembly is actuated. The discharge valve controls flow of the tank contents from inside to outside the tank through a discharge pipe.