Bedrock, till, soil, stream and beach samples have been analysed from the Loch Lomond drainage basin together with thirty sediment cores, either 6-m or 1-m long, from the lake itself. Magnetic analyses of susceptibility (chi ) and isothermal remanence (IRM) have been made on dried bulk samples and various particle-size fractions. Down-core variations in susceptibility show a series of maxima and minima which correlate with horizons of freer-and coarser-particles respectively. Magnetic susceptibility can thus be used to establish a lithostratigraphy for the lake sediments. This stratigraphy can be linked to a time scale based on palaeomagnetic, isotopic and pollen analyses. The source of the magnetic minerals in the lake could be either primary iron oxides in the bedrock or till, or alternatively secondary iron oxides developed in the soils. The possibilities of either as the major source are discussed in terms of three proposed models: a source model, an erosional/transport model and a depositional model. Variable erosion of primary magnetite explains most of the down-core, between-core and within-drainage basin changes in mineral magnetic properties.