AbstractAn intercomparison exercise was organized between seven laboratories using various isolation procedures (extraction, distillation, ion‐exchange and alkaline digestion) and detection systems (CV AAS, cold vapour atomic absorption spectroscopy; CV AFS, cold vapour atomic fluorescence spectroscopy; GC, ECD, gas chromatography electron capture detector and HPLC with CV AFS detection) for determination of methylmercury compounds in sediment sample. All certification criteria were fulfilled and therefore the value for total concentration of methylmercury compounds was certified to be 5.46 ng g−1, with a 95% confidence interval from 4.07–5.84 ng g−1. The acceptable range, calculated as two times the confidence interval of the mean is therefore from 4.68–6.23 ng g−1. This is the first sediment reference material ever to be certified for concentration of methylmercury compounds. Comparison of the data obtained by various methodologies has shown that the most critical step is the isolation of methylmercury compounds from binding sites. Acid leaching only cannot release methylmercury compounds quantitatively. Total release of methylmercury compounds could only be achieved by alkaline digestion or distillation. This simple intercomparison exercise has shown that since large numbers of laboratories world‐wide are performing methylmercury compound analyses using various improved and specific separation methods and sensitive detection systems, certification of methylmercury compounds in different biological and environmental samples should not be a problem in the future.