For analytical methods to be applied to international safeguards, precision and accuracy must be well established. With this objective an interlaboratory measurement evaluation program— “IDA-80,” which determined the elemental and isotopic content of the input solutions to reprocessing plants—was carried out with the participation of 33 laboratories from 15 countries or international organizations. It was guided jointly by the Central Bureau for Nuclear Measurements (CBNM) and the Kernforschungszentrum Karlsruhe (KfK) under the auspices of the European Safeguards Research and Development Association and with the support of the International Atomic Energy Agency. The element concentrations and isotopic compositions of all test materials were characterized by CBNM and the U.S. National Bureau of Standards to a high accuracy. The evaluation of more than 60000 analytical data reported by the participating laboratories yielded detailed estimates of the isotopic measurement capability of the laboratories for uranium and plutonium isotopes as well as for uranium and plutonium element assay by isotope dilution mass spectrometry. It also identified a number of sources of error. Compared to the results obtained in the “IDA-72” interlaboratory experiment—a similar program organized earlier by KfK with the participation of 22 laboratories from 13 countries or international organizations—considerable improvement of isotope dilution analysis over the last decade is shown.