Summary: It was found frequently that in CO2-rich waters measurements of δ13TDC are inconsistent with carbonate chemistry and δ13TDC is mistakenly considered as δ13 HCO3−. Because δ13TDC values are PCO2 dependent, it is proposed that all δ13TDC measurements in CO2-rich waters below boiling point are to be made with reference to f. example 0.1MPa of PCO2 and 25 degrees centigrade. PCO2 may be calculated from pH and alkalinity measurements or measured directly. δ13TDC at the reference pressure may be recalculated using Raleigh equations in the form presented by Wigley et al. (1978).