In the framework of an international certification campaign, organized by the European Commission, inductively coupled plasma isotope dilution mass spectrometry (ICP-IDMS) was used for the determination of Cu in two groundwater candidate reference materials. No sample pre-treatment other than addition of (i) a65Cu-enriched tracer (for isotope dilution purposes) and (ii) an internal standard (to accurately correct for the blank) was carried out. As a result of the presence of molecular ions of the same nominal mass as the analyte nuclides of interest, especially for the water showing the lowest Cu content, a double focusing sector field instrument, operated at a higher resolution setting (R=3000), was used in order to avoid biased isotope ratios and hence erroneous results. Despite the relatively low level of Cu (about 2.5 µg l–1) in one of the materials involved, the results obtained by ICP-IDMS are in excellent agreement with those obtained by other techniques in operation at other laboratories.