Phreatic eruptions often happen at crater lakes sitting on top of active volcanoes. This article describes geochemical work that has been done at some crater lakes with an intention to find the processes and precursory signals for phreatic eruptions using mainly sulfur species dissolved in lake water. When magmatic SO2 and H2S are supplied to the bottom of a crater lake, polythionates (S4O62−, S5O62−, S6O62− etc.) form. Their relative abundance and changes in concentration reflect the supply rate of the gases, and thus can be used to infer the magmatic-hydrothermal system underlying the lake. In this chapter, application of polythionate speciation and changes in the concentration are reviewed, focusing mainly on the Yugama crater lake in Kusatsu-Shirane volcano, Japan, including other crater lakes in the world. The temporal variation in sulfur and oxygen isotopic compositions of SO42− dissolved in crater lakes was also reviewed for Yugama, Ruapehu (NZ), Poás (Costa Rica), Kawah Ijen (Indonesia), and El Chichon (Mexico). The isotopic variations are influenced by thermal conditions in the vent systems. Attainment of sulfur isotopic equilibrium during the SO2 disproportionation reaction was evaluated. Isotopic fractionation between \({\text{HSO}}_{4}^{ - }\) and elemental sulfur as the products of the SO2 disproportionation reaction is controlled by temperature in the vent system. Some crater lakes contain \({\text{HSO}}_{4}^{ - }\) produced by atmospheric/bacterial oxidation of elemental sulfur and H2S at the lake surface. Participation of such sulfate disturbs the sulfur isotopic thermometry, and therefore discussion on the sulfate of surface origin was excluded.