Fish death observed in mixing zones of rivers with different acidities and elevated levels of aluminium was the subject of an environmental study in which five European countries participated. High levels of aluminium are considered to be the main cause of the fish death in these mixing zones. Therefore, a reliable determination of Al and other trace elements in gills from freshwater fish was required. Both INAA and ICP-MS were studied as candidate reference methods. Because of their sensitivity, specificity and flexibility, INAA and ICP-MS seem to be suitable methods for the determination of trace elements. INAA requires a minimum sample handling with a small risk of contamination and no blank value, whereas ICP-MS requires dissolution of the sample material entailing a considerable risk of contamination and a significant reagent blank. A comparison of results is presented using both methods. In connection with this comparison INAA must be considered here to be a quality control. The verification of results obtained by ICP-MS is therefore based on the previous validation of the INAA-method.