Sediment has been considered to be one of the most important mercury methylation sites, but recent studies have demonstrated a new site that is relevant, i.e. the roots of floating aquatic macrophytes, where high methylation is observed. The effects of temperature, pH and electric conductivity on net mercury methylation were studied in the roots of the waterhyacinth Eichhornia crassipes of a freshwater coastal lagoon (Lagoinha, RJ, Brazil). Root samples were incubated, over three days, with 203 HgCl 2 addition, at different temperatures (10-90 °C), pH values (3-8) and different electrolytic solutions (KClO 4 , KCl and CaCl 2 , at 1, 5, 10, 25 and 50 mM, ranging between 18 and 760 mS cm À1 ). 203 Hg-labeled methylmercury (Me 203 Hg) was extracted in toluene, after acid leaching, and measured by b-counting. Up to 35% of mercury added was converted to MeHg Methylation increased from 10 to 35 °C, and decreased thereafter. The process was completely inhibited at 90 °C. At pH values of 6 and 7 methylation was stimulated and a significant decrease was verified at pH 8. Increasing KClO 4 concentrations led to a significant decrease in the methylation rates, while for KCl and CaCl 2 solutions only a slight decrease was observed.