Ionic composition, salinity, temperature, pH, tritium, and stable isotope compositions of water and ions were determined for samples collected in 1969 and 1982 from different depths of stratified Lake "A" on Ellesmere Island. Tritium contents and ionic and stable isotope compositions were diagnostic of recent fresh water overlying older, deeper trapped seawater. A temperature maximum occurs at 15 m in the freshwater–seawater transition zone. Salinity, δ18O, and δD data suggest that the lake evaporated slightly and acquired about 12% fresh water prior to stratification. Individual ion concentrations reveal a slightly modified cation composition and marked depletions in sulphate and enrichments in dissolved carbonate compared with modern ocean water. The δ34S, δ18O, and δ13C data for and attest to extensive anaerobic reduction during the lake's history.