This International Standard provides methods for :- determining the composition of a calibration gas mixture by comparison with appropriate refernce gas mixtures,- calculation the uncertainty of the composition of calibration gas mixtures in realtion to the known uncertainty of the composition of the reference gas mixtures with which they are compared- validating the composition attributed to a calibration gas mixture by compartison with appropriate reference gas mixtures- comparing the composition of several calibration gas mixtures, e.g. for the purpose of comparing different methods of gas mixture preparation, or of testing consistency among gas mixtures of closely related composition.