Abstract <p>The hydrothermal systems of the Kuril Islands with horizons of ultra-acid waters of SO₄‒Cl or Cl‒SO₄ composition are associated with active volcanoes characterized by complex structures and phreatic/phreato-magmatic activity. Depending on the filtration and discharge conditions, three groups of ASC waters are distinguished, each of which is characterized by a specific set of trace elements, the distribution of transfer (enrichment) coefficients, and the behavior of rare earth elements. The classical-type waters are enriched with volatile elements (B, As, Se, Te, Cd, Pb, Ge, Tl). High-charge, large-ion lithophile, and rare-earth elements have enrichment coefficients close to unity in these waters, which corresponds to their complete transition from rock to solution. Diluted waters are characterized by low (less than unity) enrichment coefficients of most elements due to the deposition of minerals along the water filtration pathway, and their REE distribution is characterized by the simultaneous depletion in light and heavy elements with an insignificant Eu minimum. Mixed waters are maximally enriched in volatile (B, Zn, Se, Cd and As, Te) and rare alkali elements (Li, Rb, Cs), and are depleted in light REE.</p>