This International Standard specifies procedures to be used for the calibration of mass-spectrometer-type leak detectors. lt deals only with leak detectors which have an integral high vacuum System to maintain the sensing element (mass spec- trometer tube) at a low pressure. Specifically excepted from treatment are sensing elements without such a vacuum System. lt is also to be understood that the procedures are not intended to constitute a complete acceptance test; such tests will be the subject of a future International Standard. This International Standard concerns the use of helium-4. Nevertheless, the procedures described may be used for other search gases such as argon-40, subject to appropriate precau- tions. The application of this International Standard is restricted to leak detectors not capable of detecting leaks smaller than 10-12 Pa.m3s-1. Two procedures are outlined, one for determining the minimum detectable leak rate and the other for determining the minimum detectable concentration ratio. These are applicable to the use of the leak detector for high vacuum and for pressures greater than atmospheric, respectively.