Bioaccumulation of methylmercury (MeHg) in boreal systems is significant, leading to widespread fish consumption advisories. Forest harvesting may increase MeHg exports from watersheds, with dissolved organic matter playing an important role in MeHg transport. It remains unclear, however, the role that organic matter composition plays in MeHg production and accumulation, especially in watersheds disturbed by forest harvest. We compared results of total mercury (THg) and MeHg concentrations, Hg methylation and MeHg demethylation potentials (Kmeth and Kdemeth) and carbon and nitrogen contents in soils and sediment between a harvested and a reference unharvested watershed located in Northwestern Ontario. The composition of water-extractable organic matter (WEOM), which is the most soluble and reactive fractions of organic matter in soil and sediment, were examined using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). In sediment, THg and MeHg concentrations were consistent between watersheds, but total carbon content, Kmeth and Kdemeth were significantly higher in the reference watershed. Soils of vegetated buffer areas, which include wetland, in the harvested watershed had significantly higher MeHg concentrations and a greater proportion of THg present as MeHg (%-MeHg). Dissolved organic carbon and MeHg in WEOM solutions were more abundant in harvested soils. Analysis of molecular composition of WEOM is ongoing, but thus far, results suggest that harvesting activities at this site did not significantly influence MeHg production in stream sediment, but did enhance Hg methylation and MeHg accumulation within the buffer areas. Water-extractable organic matter plays a role in affecting MeHg production in these soils.