Abstract. Since 1977, seven national metrology institutes (NMIs) have submitted eleven samples of known activity of ²⁴¹Am to the International Reference System (SIR) for activity comparison at the Bureau International des Poids et Mesures (BIPM). The activities ranged from about 1.9 MBq to 17 MBq. The results of the CCRI(II)-K2.Am-241 comparison held in 2003 with twenty-one participants have been linked to the BIPM.RI(II)-K1.Am-241 results using the submission to the SIR by the NPL of two ampoules of the mother solution of the CCRI(II) comparison. This has enabled four NMIs to update their earlier SIR results and fifteen NMIs, the IRMM and the BIPM to have their result published, the previous international comparison of ²⁴¹Am having been held over 40 years ago. The results of a COOMET.RI(II)-K2.Am-241 comparison held in 2006 have also been linked to the SIR results. This is possible through the submission to the SIR of an ampoule of the comparison by the pilot laboratory, the VNIIM. This has enabled two further NMIs to have degrees of equivalence for activity measurements for this radionuclide. The key comparison reference value (KCRV) has been updated to include the recent NPL and VNIIM results. The degree of equivalence between each equivalent activity measured in the SIR, or linked to the SIR, and the KCRV has been calculated and the results are given in the form of a matrix. A graphical presentation is also given. Degrees of equivalence between pairs of participants are similarly given as a matrix.

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The final report has been peer-reviewed and approved for publication by the CCRI(II), according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).