The surficial bottom sediments of Loch Tummel essentially comprise poorly sorted, polymodal, quartzose silts and sandy silts of high water content (17–87%, mean 69.5%) and wchich are rich i organic matter (L.O.I. 0.8–29%, mean 14.5%). Their polymodality is a reflection of derivation mainly from local tills but appreciable quantities of opaline silica (8–12%), i the form of diatomaceous debris, may contribute to the polymodal character. A mean of 53.6% of the organic matter is composed of organic carbon while some 22.5–91.6% is in the form of extractable humic matter. The distribution of bottom sediments in terms of texture, Mz f and s1f, is similar to that of many lakes in that it conforms to an inshore-offshore progradation associated with declining energy as water depth increases. Less regular distributions of Sk1 and KG are a function of sediment polymodality. Markedly different textural data are obtained when grain size analyses are performed with and without the removal of organic matter. These suggest that organic matter forms “coatings” on grains of all sizes, but is predominantly adsorbed by fine particles causing them to be bound into larger aggregates which are resistant to dispersant action.