Pore waters extracted from sediment cores were analyzed for their oxygen and hydrogen isotopic compositions and major ion chemistry to determine the source of water from a vent area for diffuse lake-bottom thermal springs or seeps in Frolikha Bay, northeastern Lake Baikal. The δ18O values of pore waters range from -15.2‰ to -16.7‰, and δD values range from -119‰ to -126‰ (both isotopes determined relative to standard mean ocean water [SMOW]). Bottom water in Lake Baikal has a δ18O value of -5.6‰ and a δD value of -120‰ Pore waters in the vent area are significantly enriched in Mg, B, Ca, and especially Na and have the lowest δD and δ18O values; these pore waters are isotopically and chemically distinct from pore waters in other, more typical parts of the lake. The pore-water isotopic data fall on a local meteoric water line, and covariations in water isotopes and chemistry are not consistent with evaporation or hydrothermal water-rock interaction. The thermal springs represent discharging meteoric waters that have been gently heated during subsurface circulation and are largely unaltered isotopically. Chemical variations are most likely due to dissolution of subsurface evaporites.