1-Octanol/water partition coefficients, [Hg11]octanol [Hg11]water, provide a simple but limited model system for aspects of the biological behavior of methylmercury(II) and commonly used organomercury(II) medicinal compounds. In an octanol/water system some widely studied antidotes for mercury poisoning at least partly displace the biological thiols L-cysteine and glutathione from binding to MeHg11 at pH 6.9. Addition of the antidote meso-dimercaptosuccinic acid to MeHg11 in the presence of glutathione results information of metallic mercury. For RHg11 derivatives of L-cysteine and glutathione, octanol/water partition coefficients follow the order Ph > Et > Me. An exceptionally high value for diphenylmercury, compared with PhHg11 derivatives of L-cysteine and glutathione, is consistent with reported results of the distribution of mercury compounds in rats. Ethylmercury(II) is partly displaced from thimerosal by L-cysteine and glutathione in the octanol/water system, indicating that the active form of thimerosal in vivo may involve binding of EtHg11 to biological ligands.