The reference solutions used for the standardization of analytical methods are the means through which the final result of the analysis is traceable to the basic units and to their standards. The strength of these solutions should therefore be traceable to these same units with a relative accuracy of + 0,l % or better. This International Standard specifies procedures for preparing reference solutions, as well as a set of procedures for ensuring the quality of their preparation and the degree of accuracy of the solution strength. It is intended to be a working document for analytical chemists. The underlying statistical theory is therefore not presented in full details or rigorously, in order to simplify application. Sufficient details are included in annex H, in order to implement the procedure step by step in a laboratory en- vironment.