Research was conducted to clarify the mechanism of removal of aquatic humus by coagulation with aluminum and to determine the optimum coagulant dosage and pH for maximum removal of humus. Three different raw waters, lakes Tjernsmotjern, Hellerudmyra, and Smaputten, were studied. These raw waters varied in respect to concentration of total organic carbon (TOC) and in respect to the molecular size distribution of the organics. In all cases a pH of 5.5 was the optimum for efficient coagulation. Water sources with the greatest amount of low-molecularweight organics were shown to have the highest coagulant demand and the lowest removal of organic carbon. When sufficient alum was added for maximum removal of humus at the optimum pH, residual aluminum was minimized. Includes 47 references, tables, figures.