This International Standard specifies a method of determining whether an analytical system for natural gas is satisfactory, on the assumptions that a) the analytical requirement has been clearly and unambiguously defined, for the range and uncertainty of component concentration measurements, and the uncertainty of properties which may be calculated from these measurements; b) the analytical and calibration procedures have been fylly described; c) the method is intended to be applied to gases having compositions which vary over ranges normally found in transmission and distribution networks. If the performance evaluation shows the system to be unsatisfactory, all the stages, such as - the analytical requirement; - the analytical procedure; - the choice of equipment; - the choice of calibration gas: - the calculation procedure; must be re-examined in the light of the test data to assess where improvements can be obtained. This International Standard is applicable to analytical systems which give the component concentrations and resulting uncertainties. With the present state of kowledge, the method chosen is likely to be gas chromatography. Performance evaluation of an analytical system should be performed during installation, then at regular intervals, according to the application, and/or whenever any critical component of the analytical system is changed or replaced.