Peatlands long have been considered to preserve the record of atmospheric deposition of anthropogenic contaminants such as Pb. In the past two decades, 210Pb has been widely used to date recent strata of peat and to calculate accumulation rates. The assumption that Pb and 210Pb are immobile and not subject to diagenesis in peat has been questioned but not rigorously tested. We attempted to determine if Pb is mobile in peatlands and if Pb profiles are altered by diagenic processes by constructing a mass balance for Pb about a small peatland, by comparing inventories, concentrations and accumulation rates of Pb and 210Pb in peatlands across northeastern North America and by examining the relationship between concentrations of Pb in bog waters and peat in numerous sites. Our results clearly demonstrate that Pb and 210Pb are mobilized by the organic-rich waters of peatlands. Profiles of Pb and 210Pb at depths below the water table do not preserve the record of atmospheric deposition and inventories of Pb and 210Pb are depleted in peatland hollows. Concentrations of Pb in bog waters are regulated by the concentration of Pb in the peat and the concentration of dissolved organic carbon. The mass balance for one bog indicated that over the specific three-year period of study more than 30% of inputs of Pb were not retained within the peat. As a result of this mobility, dates based on 210Pb can be biased and inaccurate by as much as 30 years. Dates based on 210Pb should be verified by other techniques, especially when the inventory of 210Pb is less than that expected from local rates of deposition.