Key comparison CCQM K-28 was undertaken to assess the measurement capabilities for quantitation of (C4H9)3Sn+ (TBT) in a prepared marine sediment by National Metrology Institutes (NMIs), which are members of the Comité Consultatif pour la Quantité de Matière (CCQM). It follows a previous pilot study, CCQM-P18. [1, 2] This exercise was sanctioned by the 8th CCQM meeting, 18–19 April 2002, as an activity of the Inorganic Analysis Working Group and was jointly coordinated by the Institute for National Measurement Standards of the National Research Council of Canada (NRC) and LGC, UK. Eight NMIs initially indicated their interest, with seven ultimately submitting their results. All NMIs relied on isotope dilution mass spectrometry using a species-specific 117Sn-enriched TBT standard, which was supplied by LGC. No analytical methodology was prescribed for this study. As a result, a variety of extraction approaches was adopted by the participants, including mechanical shaking, sonication, accelerated solvent extraction, microwave assisted extraction and heating in combination with ethylation and direct sampling. Detection techniques included ICP-MS (coupled to GC or HPLC for the separation of Sn species) and GC-MS.