The structure, origin and temperature regime of caves located within the Lena Pillars National Nature Reserve in the middle stream of Lena River (Yakutia, Eastern Siberia), have been considered. Specific features of the configurations of “warm” and “cold” caves largely influencing air circulation and seasonal changes in the thermal regime are discussed. The first ever data are provided on the isotopic composition (18О and D) of various types of hoar ice, allowing to reliably discriminate them between the different types of surface and ground ice in the area. It has been established that the moisture of cave ice is sourced from the warm season precipitation. A horizontal zoning was revealed in distributions of hoar ice crystals and their isotopic composition. Columnar crystals of extremely heavy isotopic composition was found to form in the internal zones with temperature averaging about –8 °C (δ18О = –(12.2 ± 0.7) ‰, δD = –(99.2 ± 4.7) ‰, dexc = –2.0 ± 0.8), while plates with spiral structure formed in the transition zone have the following composition: δ18О = –(14.9±1.6) ‰, δD =
= –(118.3 ± 12.0) ‰, dexc = 1.0 ± 0.9. The lightest composition (δ18О = –(21.2 ± 0.8) ‰, δD =–(178.0 ± 4.7) ‰, dexc = –8.2 ± 1.5) has been established for hoar ice forming a belt of fine-crystalline hoarfrost near a cave entrance.