The production of methylmercury � a highly bioaccumulative and biomagnifying neurotoxin - in saturated soils and sediment, and its movement into surface waters are key processes regulating the amount of mercury available for uptake into aquatic food webs. Our recent work across nineteen small catchments encompassing a diversity of landscape features in northwestern Ontario suggests that methylmercury production in and mobilization from stream sediment are the dominant processes leading to variation in stream water methylmercury across these same catchments. Methylmercury production rate constants (Kmeth) in stream sediment are significantly greater than in wetland or riparian soils, whereas demethylation rate constants (Kdemeth) are statistically similar across soils and sediment. Additionally, the proportion of soil or sediment methylmercury that was water-extractable, and likely more mobile, is significantly greater in stream sediment compared to wetland or riparian soil. Using high-resolution mass spectrometry, we also found significant molecular differences in molecular composition and organic matter lability across soil and sediment, and overall found that the proportion of labile water-extractable organic matter was strongly (R2 = 0.84) and positively correlated with water extractable methylmercury concentrations. These findings do not necessarily discount the importance of wetlands and/or wet soils to methylmercury loading in streams. Stream sediment, however, may be more important than previously considered, and possibly dominant, as a source of methylmercury to stream water and downstream systems.