By mixing aliquots of pure Na 2 CO 3 and CO 2 , solutions with precisely known molal fractions of CO = 3 , HCO - 3 , and dissolved CO 2 were prepared. The apparent fractionation factor between gaseous CO 2 and total carbon in the solution was determined mass spectrometrically from which the CO = 3 - CO 2 fractionation factor was calculated taking into account the known fraction of HCO - 3 and respective isotope fractionation. Although the measurements have been made for a rather narrow temperature range, from 4 to 80°C, a theoretical curve was fitted through the experimental points, and thereby, the isotopic partition function ratio of 1 3 CO = 3 and 1 2 CO = 3 molecules has been obtained for an extended temperature range. The results obtained for CO = 3 -CO 2 isotope fractionation are significantly lower than those for HCO - 3 -CO 2 exchange (with 10 3 (α - 1) = 5.0 ± 0.2 at 25°C and cross-over point at about 63°C).