This International Standard describes factors that may influence the composition of pure gases and homogeneous gas mixtures used for calibration purposes. This International Standard only applies to gases or gas mixtures that are within the “utilization period”, and it pays special attention to -- storage of calibration gas cylinders; -- calibration gas withdrawal from cylinders; -- transfer of calibration gas from cylinders to the point of calibration. It also outlines a method of assessing the stability for a gas mixture, and takes into account the gas composition uncertainty given on the certificate and the users measurement uncertainty.