This International Standard specifies a method of isotopic analysis of uranium hexafluoride (UF6) with 235U concentrations less than or equal to 5 %1) by mass and 234U and 236U concentration between 0,001 % by mass and 1,5 % by mass. The method is in routine use to determine conformance to UF6 specifications. Uranium hexafluoride is a basic material to prepare nuclear reactor fuel. To be suitable for this purpose, the material must meet criteria for isotopic composition. This International Standard is applicable if the mass spectrometer uses Faraday cups.