Thin-layer chromatographic methods for the separation and identification of organomercury compounds have been developed and applied to the examination of commercial-quality organomercurials and seed-dressing formulations. The organomercury compounds, in the form of their chlorides, are separated on silica gel by continuous development with cyclohexane-acetone (4 + 1), and detected as orange-brown spots by spraying the plate successively with copper sulphate and sodium sulphite-potassium iodide reagents. If organomercury salts other than chlorides are present, they must be converted into the chlorides by adding aqueous sodium chloride solution to the spotting solvent.A method for the semi-quantitative assay of inorganic mercury [as mercury(II)] in the compounds, at levels between 1 and 14 per cent., is also described. The inorganic mercury [as mercury(II) chloride] is separated by using carbon tetrachloride-acetone (7 + 3) as the developing solvent and is determined by visual comparison with standards.Little or no mercury-containing impurity was found in samples of phenylmercury nitrate, phenylmercury acetate and phenylmercury chloride examined, but methoxyethylmercury chloride and ethoxyethylmercury chloride samples contained as many as five mercury-containing impurities and up to 12 per cent. of inorganic mercury compounds.