—We consider the factors determining the mineral composition of hydrothermal clays in recent thermal anomalies of the Pauzhetka–Kambal’nyi–Koshelevskii region and Ebeko (Paramushir Island) and Mutnovskii Volcanoes. The composition of mineral assemblages is influenced by a number of interdependent factors: the type of discharge (steam–gas jets, steaming ground, and mud water pools), temperature, pH, and Eh of a thermal solution. Mineral assemblages including sulfur, opal, α-quartz, and alunite group minerals form on the steaming ground of the craters of active volcanoes (Ebeko and Mutnovskii). Mud water pools of the craters contain mainly sulfur, opal, and α-quartz. An assemblage of kaolinite, opal, alunite group minerals (in small amounts), and goethite forms in mud water pools with pH = 2–3 in the thermal fields of the Pauzhetka–Kambal’nyi–Koshelevskii region. An assemblage of newly formed kaolinite, smectite, pyrite, marcasite, and, sometimes, opal is specific to mud water pools with pH > 3.5–4.0 At pH > 5, smectite, pyrite, and opal are produced. In the thermal fields of the Pauzhetka–Kambal’nyi–Koshelevskii region, the walls of steam–gas jets that are often flooded with water and dry out are formed by kaolinite, smectite, opal, α-quartz, pyrite, marcasite, and goethite. The walls of steam–gas jets that are not flooded with water for a long time consist of kaolinite, opal, α-quartz, smectite (traces), alunite group minerals, pyrite, and marcasite in close amounts. The steaming ground of the thermal fields of the Pauzhetka–Kambal’nyi–Koshelevskii region is composed of kaolinite, alunite group minerals, opal, and goethite or an assemblage of kaolinite, smectite, opal, pyrite, and marcasite.