Abstract <p>The results of multiyear observations were used to characterize the volcanic hydrothermal systems containing horizons of hyper-acid waters of SO₄–Cl or Cl–SO₄ composition (ASC-waters) on five islands (Paramushir, Shiashkotan, Urup, Iturup, and Kunashir). We used physicochemical parameters and the macrocomponent composition of the thermal waters to develop an additional classification. Depending on the conditions of percolation and discharge we identified three groups of ASC waters: (a) classical of Al–Fe–SO₄–Cl composition, (b) those diluted with groundwater of Ca–Cl–SO₄(SO₄–Cl) composition, and (c) those mixed with deep thermal waters of Na–Cl–SO₄ composition. These groups differ in pH, temperature, salinity, and in the SO₄/Cl/F and (Na + K)/(Ca + Mg)/(Al + Fe) ratios. The first group of classical hyper-acid waters of SO₄–Cl composition includes the springs at Ebeko Volcano (Paramushir) and at Baransky Volcano (Iturup) whose cation compositions are dominated by Al and Fe. The second group, which is diluted with cold groundwater, includes the thermal waters discharged on the slopes of Sinarka Volcano (Shiashkotan) and Berg Volcano (Kunashir). The third group which is mixed with sodium chloride waters includes the springs at Mendeleev and Golovnin volcanoes (Kunashir). The cation composition of all ASC waters studied on the Kuril Islands is equivalent to less than 10 grams of rock dissolved in one liter of water.</p>