Data on temperature, water chemistry, and abundance of bacteria in Ace Lake, Antarctica are presented. In particular, the sulfur chemistry and microbiological fractionation of sulfur isotopes are discussed in relation to the possible development of the lake and the climatic history of the lake catchment. The lake is covered by ice for 9 months each year and has a maximum depth of 23 m. A layer of water at a 10‐m depth contains colloidal sulfur and is situated between a well‐oxygenated mixolimnion and an anoxic monimolimnion. In the monimolimnion, H2S concentrations rapidly increase with depth and SO42‐decreases to below the limit of detection near the bottom. High δ34S values (up to +67‰) were encountered in sulfate from the lake and mean δ34S values of sulfate plus sulfide were similar at all depths measured.