The Bureau International des Poids et Mesures (BIPM) is developing, with the support of national metrology institutes, an extension to the existing international reference system for international comparisons in radionuclide metrology. The extended system is addressing pure alpha-, pure beta-emitting and some electron capture-decaying radionuclides. The new system is based on a custom-built liquid scintillation counter and the triple-to-double coincidence ratio (TDCR) technique. Particular emphasis was placed on long-term stability and excellent reproducibility in sample preparation. The analytical method is based on a special approach that uses the TDCR value as a parameter to compensate for fluctuations caused by changes in detection efficiency or photomultiplier asymmetries that cannot be excluded when considering very long periods. Measures for quality assurance were established. They are based on periodic controls of unquenched toluene-based liquid scintillation sources with .sup.3H and .sup.14C. In this way, the reproducibility of the system results is demonstrated for a test period of about twenty months.