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 P_CO2 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 P_CO2 and 25 degrees centigrade. P_CO2 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).