The preparation is described of a batch of 1100 vials containing a lyophilised solution, with CrVI and CrIII contents as in natural or tap water and a total Cr content <50 µg l–1 following the European Community Directive 80/778/EEC, L229/20, D48 on the quality of drinking water. The homogeneity and stability studies and the analytical work performed for the certification of the contents of the CrIII and CrVI species and the total Cr content are also described. The paper contains all the results and gives the methods used to certify the measured concentrations of CrVI (22.79 µg l–1), CrIII (26.77 µg l–1) and total Cr (49.34 µg l–1) after reconstitution in 20 ml. A brief description of the preliminary feasibility study is also given.