Describes the quality control activities in the field of trace element determination in fossil fuels (coal and fuel oil). The determination of As, Hg and Se in coals was carried out by different techniques together with the determination of several trace metals in residual fuel oils. The use of certified reference materials in order to check the accuracy of procedures is discussed and a report made on the results obtained for NIST 1632a and NIST 1632b (coal samples) and NIST 1634b and NIST 1619 (fuel oil samples).