Abstract Loch Bradan, a drinking water reservoir in SW Scotland, frequently exhibits unacceptably high dissolved Mn concentrations. Both the surrounding catchment and the loch sediments are potential sources of Mn to the loch water. This study focused on the catchment soils, which are peaty, and found that redox cycling was an important process with respect to retention of Mn in the top sections (0–15 cm). Under more reducing conditions, reduction to Mn(II) and subsequent complexation by humic substances was observed at greater depth in some soil profiles. Complexation by humic substances is important because lateral water flow can remove soluble complexes and indeed this study observed that about 50% of Mn was humic-complexed in the stream waters feeding into the loch. It was particularly evident that the soil profile with the lowest Mn inventory exhibited the greatest extent of humification and that the remaining Mn was predominantly in a non-easily reducible form.