Abstract <p>The study provides the first data on the quantity and distribution of stable isotopes of boron (δ¹¹B) and chlorine (δ³⁷Cl) in waters of the Sinegorskoe field of high pCO₂ mineral water and the Yuzhno-Sakhalinsk mud volcano (Sakhalin Island), in addition to chemical compositions and isotopy (¹⁸O, D, and ³H). It is shown that the investigated waters are attributed to formation waters, and variations in their chemical composition and isotope characteristics (³H) are related to the water–rock–gas interaction, with the insignificant influence of atmospheric precipitation. Measurements of chlorine and boron isotopes performed for the first time allowed us to establish that waters in both cases formed as a result of interaction between water and rocks of marine genesis, with no involvement of juvenile or subduction waters, as well as volcanic gases. The source of chlorine in the waters is evaporites of Mesozoic and Cenozoic marine sedimentary strata. Clay alteration processes (illitization of smectite) are the source of boron in the water, according to the obtained δ¹¹B and δ¹⁸O values. It may well be that marine borates are an additional source of boron in Sinegorsk mineral waters. The observed variations in isotope values of boron and, to a lesser extent, chlorine in mineral and mud volcanic waters are explained by significant differences in geological conditions of the studied objects. Based on the isotope-geochemical data, a scheme is proposed for the formation of the isotope-chemical composition of waters of the Sinegorskoe field and the Yuzhno-Sakhalinsk mud volcano.</p>