Abstract Extracts of humic substances in river sediment and leaf mould have the ability to methylate mercury; several compounds have been identified by GC-MS. The methylating abilities of the compounds found and known as constituents in humic material were determined. Only three compounds, 2, 6-di-tert.-butyl-4-methylphenol (BHT), p-xylene and mesitylene had the ability to methylate at pH 3.5. At pH 7, only BHT produced methylmercury, the other two compounds did not. Methylmercury production by BHT increased with increase of temperature, mercury concentration and BHT concentration, and was also influenced by pH; a high production rate was observed at pH 2 and 13.5. BHT was found in river sediment and leaf mould, and the possibility of a contribution to methylmercury formation by BHT in the environment is indicated.