The study of the carbon isotopic composition in organic-bearing bottom sediments of the thermal lake Fumarolnoye (Uzon caldera, Kamchatka) has been carried out. The results of studying δ13C changes in the bottom sediments of a thermal lake during the sequential extraction of H3PO4 for 2 hours, 24 hours and 7 days are presented. The change in δ13С with time and after treatment with HCl and H2O2 is shown using two samples as an example. After treatment with HCl, the δ13С values change relative to the initial ones from 1 to 3 ‰, which can be taken into account when interpreting the data. The standard procedure for removing carbonate with hydrogen peroxide reveals a significant transformation of the residual C‑containing substance in the case of a prolonged reaction with H3PO4 at elevated temperatures