These data include the measured pHt, total alkalinity, dissolved inorganic carbon, raw pHt absorbance, and calculated pHt of 25 batches of seawater as a function of temperature, salinity, and pCO2. These data were used to evaluate the internal consistency of 120 different possible combinations of CO2 system constants. The marine inorganic carbon system can be calculated with two measured parameters due to thermodynamic relationships. However, there are many different parameterizations for the required constants and the most accurate or best is not known. These data were used to evaluate the constants and make recommendations for which constants to use, and how to perform CO2 system calculations.