The invention relates to a closure system on a fuel tank (2) of thermoplastics with barrier properties for hydrocarbons. The closure system comprises at least one orifice (4), with a closure cap (5) which closes the orifice (4) in a sealing manner and which cooperates in a form-fitting manner with closure means provided at the circumference of the orifice (4) and encloses at least one first sealing means between it and a tank wall (3). The closure system (1) according to the invention is distinguished in that the tank wall is sealed in the region of the orifice (4) with at least one second sealing means, which covers and/or shields a part of the tank wall surfaces exposed to hydrocarbons.