Biomethylation involves the addition of a methyl (-CH3) group to a metal such as arsenic, mercury, cadmium or lead. The methylated forms are more mobile and can migrate into the groundwater. Although, methylation increases volatility, it is not likely that methylation of metals such arsenic will be performed for remediation since the by-products are more toxic. They are currently under develop- ment and not commercially available. Volatilizationof selenium from contaminated agricultural soils has shown some promise (Thompson-Eagle and Frankenberger, 1990) Therefore, these processescould be used for soil and sediment treatment. How- ever, this form of the metal may difficult to control in gas emissions.Another process (called mercrobes) has been devel- oped and tested in Germany at concentrations greater than 100 ppm. Between 95 and 99% of the mercury was reduced in laboratory tests (Hazardous Waste Consultant, 1996). Since the mobility is influenced by its oxidation state, these reactions can affect the contaminant mobility