Crater lakes with active subaqueous fumaroles often contain molten sulfur pools on the lake floor. Volcanic gases passing through the sulfur pools carry hollow spherules of solidified molten sulfur to the surface of crater lakes. This sulfur dissolves SO2 and H2S gases and releases these gases into the water. The sulfur also contains homocyclic sulfur (cycl. Sx, x = 6–16) and probably sulfane monosulfonates. The concentration of cyclic S7 increases with increasing temperature between 120 and 175°C, which is useful to estimate the temperatures of subaqueous molten sulfur pools. The gases drastically lower viscosity of the molten sulfur. This may be due to blockage of growing long-chain sulfur molecules by the dissolved gases. Thus a jump in viscosity at 159°C observed for pure sulfur is not likely to be present in subaqueous molten sulfur at crater lakes. Based on the chemistry and morphology of sulfur slicks, activity of subaqueous fumaroles can be divided into four stages (I–IV), each of which may serve for qualitative in situ monitoring of crater lakes. At Stage I, no molten sulfur pools exist on the lake floor and fumaroles discharge low temperature gases ( 150°C, resulting in an increase in molten sulfur viscosity; and at Stage IV, frequent phreatic or geyser-like eruptions are observed. The molten sulfur pools are dispersed into pieces on the lake floor at this stage.