Surface waters of the Lake Pedder area are characterized by low salinity, low pH, high colour, and a relative major-ion composition near that of seawater. Factors determining this composition are predominance of inert rocks, a covering of sedgeland peat isolating waters from rock contact, and a high rainfall brought by prevailing oceanic winds. Frequency and intensity of rainfall appear to determine the pattern of chemical variation. Salinity, bicarbonate, and pH may change rapidly during periods of high rainfall. Humic acids are important chemical constituents of the waters, and probably determine the type of biotic community which inhabits them.