Abstract: Chemical composition of volcanic soils has been studied in the Southern Kuril Islands (Iturup, Kunashir, Shikotan), which undergo recurrently ash falls of mainly basic composition upon volcanic eruptions. The total content of metals (K, Ca, Mg, Na, Ba, Cu, Co, Cd, Cr, Fe, Mo, Ni, Pb, Sc, Sr, V, and Zn) and the concentrations of their mobile forms extracted by the ammonium acetate buffer at pH 4.80 have been determined. The total contents of Sc, V, Fe, and Zn are 1.5–5 times higher than their natural abundances in the upper layer of the Earth’s crust (clarkes), whereas the contents of Cr, Ni, Sr, and Ba are 2–10 times lower. The soils show contrasting pH distribution (3.75–7.81) providing for the mobility of metals. The maximum activity of radial and lateral migration leading to radical differentiation of the genetic soil horizons by their chemical composition in different catenary positions is observed at low pH values; the coefficients of radial and lateral migration can rise up to 12 and 29, respectively. The results of factor analysis have revealed the leading role of soil-forming rocks in the formation of soil chemical composition (about 63% of sample variance); whereas humus formation, transformation of mineral and organic substances in soils, and hydrothermal activity exert a lesser effect. Soils formed on intermediate and mafic rocks of the Cenozoic volcanic massif are characterized by the Ca–Mg–Na paragenetic association; those formed on deposits of liparite–dacite complex are specified by the Ba–K–Pb–Mo association; on gabbroid rocks, by the Ni–Cr–Cu association; and on psammite, by the V–Sc–Fe–Co association. The Mo–Pb paragenesis is typical for the areas of current hydrothermal activity. The influence of placer formation on the chemical composition of soils in the coastal areas of Iturup and Shikotan Islands is shown. The distribution of mobile forms is associated with chelate-formation processes, geochemical barriers of acid–base, sorption, and redox types. Local soil pollution mainly related to the operation of motor vehicles has been identified. The areas of Krabozavodskoe and Yuzhno-Kuril’sk settlements show elevated concentrations of Ba, Sr and K due to the activities of fish processing plants; and the area of Kuril’sk city manifests elevated concentrations of Cr, Ni, Cu, Co, Zn and Pb, the source of which being the repair base. The calculated index of soil toxicity probability (MERMQ), as well as the results of biotesting for Daphnia magna and Chlorella vulgaris attest to the low soil toxicity. This is explained by the low population density and low economic development of the islands.