Oxygen isotope fractionation and equilibration kinetics between CO 2 and H 2 O have been investigated at 313 K for salinities (S) ranging from 0 to 250 g L −1 . In this range of salinity, times needed to reach oxygen isotope equilibrium between CO 2 and H 2 O increase from 4 h to 12 h. Isotopic exchanges are comparable for KCl and NaCl-like (sea salt) solutions and are described by first-order kinetic reactions with ln(k)=−8.1485(±0.0057)−0.00474(±3.87×10 −5 )S. The oxygen isotope fractionation factor between CO 2 and H 2 O increases with salinity for both sea salt and KCl solutions with concentrations ranging from 0 to 250 g L −1 according to the following equation: 1000 ln(α CO2–H2O )sea salt=37.02(±5×10 −3 )+3.96×10 −3 (±1.1×10 −4 )S−6.38×10 −6 (±4.5×10 −7 )S 2 (R 2 =0.998). The oxygen isotope analysis of seawater samples with a salinity of 35 g L −1 requires minor corrections of −0.15‰ (V–SMOW). However, oxygen isotope ratios are overestimated by 0.4‰ to 0.6‰ in the case of highly saline natural waters (100<S<250 g L −1 ). Corrections of the oxygen isotope ratios due to changes in the salinity-dependent fractionation factors between CO 2 and H 2 O must be taken into account during the study of waters sampled from salt marshes, hypersaline lakes and lagoons, or hydrothermal brines.