Mercury methylation and/or demethylation have been observed in several compartments [soil (saturated soils covered by standing water), floc, periphyton, and water] of the Everglades, a wetland with mercury as one of the major water quality concerns. However, it is still unclear which compartment is the major source or sink due to the lack of estimation and comparison of the net methylmercury (MeHg) production or degradation in these compartments. The lack of this information has limited our understanding of Hg cycling in this ecosystem. This study adopted a double stable isotope ((199)Hg(2+) and Me(201)Hg) addition technique to determine the methylation/demethylation rate constants and the net MeHg production rates in each compartment. This study improved the previous models for estimating these parameters by (1) taking into account the difference between newly input and ambient mercury in methylation/demethylation efficiency and (2) correcting the contribution of photodemethylation to Me(199)Hg concentration when calculating methylation rates in water. The net MeHg production rate in each compartment was then estimated to identify the major sources and sinks of MeHg. The results indicate that these improvements in modeling are necessary, as a significant error would occur otherwise. Soil was identified to be the largest source of MeHg in the Everglades, while the floc and water column were identified as the major sinks. The role of periphyton varies, appearing to be a source in the northern Everglades and a sink in the southern Everglades. Soil could be the largest source for MeHg in the water column, while methylation in periphyton could also contribute significantly in the northern Everglades.