Mercury contamination of fish in lakes has become a public health issue in recent years. The accumulation of unacceptable concentrations of mercury in fish occurs largely as the result of bioaccumulation of methylmercury rather than inorganic mercury through several trophic levels. Therefore, the methylmercury concentration in water is the key factor determining the concentration of mercury in the aquatic biota. Anoxic sediments are an important source of methylmercury, where the processes of methylation and demethylation govern the flux of methylmercury from bottom sediments to the overlying water column. Little quantitative information is known about the dynamics of these processes, however. This paper reported a study on determining the extent of mercury methylation and demethylation in natural and highly polluted lake sediments as affected by redox conditions which may control methylation/demethylation through influencing the microbial activity and/or the concentration of bioavailable mercury.