<p>The paper presents the results of studying the scarce iron–magnesium sulfide–hydroxides of the tochilinite–ferrotochilinite series with the ideal formula 6FeS·5Mg(OH)₂. These minerals are formed during serpentinization of ultrabasites of different nature and were described in peridotites of the Kempirsai, Khabarny, Kytlym and Uktus massifs in the Urals and the Verkhne-Iusskaya area of the Shaim petroleum region of Western Siberia. The aim of this work is to examine the poorly studied chemical composition of tochilinite, to determine its mineral associations and conditions of formation. The minerals were analyzed by optical and electron scanning microscopy, Raman and IR spectroscopy. Based on the magnesium number (<i>Х</i>_Mg), the minerals could be divided into tochilinite and ferrotochilinite. High-Mg minerals (<i>Х</i>_Mg = 0.73–0.79) are found in the Uktus massif and in the Verkhne-Iusskaya area, and low-Mg varieties (<i>Х</i>_Mg = 0.15–0.38) occur in the Khabarny and Kytlym massifs. The presence of mixed-layer phases represented by the alternation of nano-scale layers of tochilinite or ferrotochilinite with serpentine is assumed. The chromium-bearing varieties of tochilinite are noted. Mechanisms and chemical reactions leading to the formation of tochilinites during the low-temperature transformation of peridotite in the presence of water are discussed. In most cases, this is the interaction of metamorphic water with magmatic sulfides during the serpentinization of peridotite, or the influence of sedimentary or another waters containing dissolved sulfur on them. It is concluded that tochilinite served as a sulfur absorbent during early reticulate serpentinization of ultramafic rocks. Tochilinite could be used as a promising geothermometer for low-temperature ultramafic mineral assemblages.</p>