This chapter is a tribute to Bokuichiro Takano and Minoru Kusakabe for their important contributions to our knowledge of sulfur chemistry and dynamics in hyper-acid crater lakes and geothermal lakes. Hyper-acid crater lakes are perched at the summit of active volcanoes and represent the uppermost manifestation of a shallow active magma-hydrothermal system. They act as traps for strongly acidic condensates formed as sulfur-rich magmatic gases rising from depth expand and cool in the main hydrothermal upflow zone. The remarkable sulfate content of hyper-acid crater lakes is sourced to disproportionation-hydrolysis of magmatic SO2 in the upper part of the hydrothermal conduit. This reaction generates a strong, temperature-dependent sulfur isotopic fractionation, which typically produces high δ34SSO4 values. In contrast, sulfate in geothermal lakes displays much lighter sulfur isotopic compositions linked to oxidation of H2S-rich hydrothermal discharges. Polythionates are ubiquitous in hyper-acid crater lakes and are usually attributed to aqueous interaction between SO2 and H2S in the lake. Fluctuations in lake polythionate concentrations have been used to infer changes in the SO2/H2S ratio of magmatic hydrothermal inputs. However, polythionates may also originate from hydrolysis of elemental sulfur. Elemental sulfur in hyper-acid crater lakes occurs primarily as a molten body at the hydrothermal vent-crater floor interface. The origin of this material is not entirely clear; several deposition reactions are compatible with the observed range of sulfur isotopic compositions. Sulfide and sulfosalt minerals commonly occur as impurities in molten sulfur from hyper-acid crater lakes. Molten sulfur is also found in some geothermal lakes. There are still plenty of research opportunities for decoding the complex cycling of sulfur between aqueous and gaseous species and elemental sulfur in hyper-acid crater lakes. In particular, efforts are needed to track intermediate sulfur species. The role that subaqueous molten sulfur plays in modulating heat and mass transfers to the overlying lake and in trapping metals transported by magmatic gases deserves further investigations.