The Koycegiz Lake in south-western Turkey is a meromictic lake, comprised of waters of both thermal and cold-karstic origin. The lake is principally fed from surface flow, ground water recharge from alluvium and thermal discharges located at the bottom of the lake. As indicated by physical and chemical observations, the lake is composed of two hydrochemically distinct water layers. There is a transition zone between these two layers at a depth of approximately 10 m. The bottom layer, which contains more mineralized water, is recognized easily by the smell of hydrogen sulfide gas. A decrease in pH and dissolved oxygen and an increase in specific conductivity and ionic constituents are observed at the transition zone in depth profiles at all measurement sites throughout the year. The isotopic compositions of the representative observation points widely scatter and indicate the mixing of thermal and cold-karstic waters. As indicated by satellite imagery data, the morphology of the lake shores are primarily controlled by structural elements, which are mostly normal faults. Echo-sounding profiles point out that some of the fault lines, along which the lake bottom springs seem to be located, extend through the lake bottom. Current velocity observations also indicate possible sites of thermal ground water discharges located at the lake bottom.