Loch Bradan, a drinking water reservoir in the Galloway Hills, south-west Scotland, frequently exhibits dissolved Mn concentrations of greater than 50 mg l^-1, the EC maximum admissible concentration for Mn. Situated in in a mineralised area overlying greywacke bedrock and granite, the catchment is peaty and hence highly organic in nature. To investigate the underlying cause and specific nature of what, given the well-oxygenated status of the loch, are surprisingly high dissolved Mn concentrations, peat cores and input stream water from the catchment along with sediment cores, pore water and loch water from Loch Bradan were collected. The concentrations of a number of parameters in the peat cores and thinly sliced sediment samples were determined, including pseudo-total Mn, easily reducible Mn, humic substances and humic-associated Mn. Concentrations of dissolved Mn at different depths in the Loch Bradan water column, pore water and input streams were measured in filtered and subsequently fractionated (< kDa and > 1 kDa) samples. The behaviour of Mn in the Loch Bradan system is controlled by a number of factors including classic redox cycling in the catchment peat and bottom sediments of Loch Bradan with enrichments of up to 2% and 7 %, respectively, occurring in the near-surface zones. The presence of insoluble inorganic species such as carbonates and associations with Fe oxides are also important influences upon the behaviour of Mn, especially at greater depth within both the sediment and the peat. The most important interaction with respect to the high dissolved Mn in the water column is, however, the association of Mn with humic substances. In the water column 50 - 60 % of the Mn was found to be associated with organics, with these species entering the water column both from the catchment (where ~ 45 - 55 % of the dissolved Mn in input streams was found to be associated with humic substances) and from the bottom sediment (where ~ 35 % of the dissolved Mn in the surface pore waters were found to be humic associated), with the humic substances in the sediment being principally terrigenous in nature.