The solubility of calcite and aragonite in seawater was studied over a wide range of salinities (5-44 per mil) and temperatures (5-40°C) at 1 atm total pressure. The study involved measuring solubility from both undersaturated and supersaturated solutions using a closed system technique, with equilibration periods ranging from 5 days to nearly 2 years. Over 480 independent measurements were conducted, and results were compared to previous studies. The stoichiometric solubility constants for calcite and aragonite were found to be statistically equivalent to those determined by several earlier researchers. However, this study is the first long-term equilibration investigation of aragonite solubility in seawater. The study used thermodynamic solubility products of calcite and aragonite and combined them with recent methods for estimating total ion activity coefficients in complex electrolyte solutions to analyze the data. The findings indicate that the influence of coprecipitates in equilibrium or solubility-controlling phases on the solubility behavior of calcium carbonate cannot be discerned within the present precision (approximately 5%). The study highlights the importance of equilibration time in determining the solubility of aragonite in seawater, as previous investigations may have failed to reach equilibrium. The results show that the temperature coefficients for both calcite and aragonite are small and negative at all salinities. The stoichiometric solubility constants of calcite and aragonite in seawater were determined and compared to previous studies, with the results showing good agreement for calcite but significant differences for aragonite. The study also discusses the importance of ion pairing in the calculation of free ion activity coefficients and the impact of these factors on the solubility of calcium carbonate in seawater. The results indicate that the solubility of calcite and aragonite in seawater closely follows the predicted solubility of pure calcium carbonate, despite the presence of mixed carbonates in equilibrium with seawater. The study concludes that recent refinements in estimating activity coefficients in complex electrolyte solutions have reached an accuracy where the use of stoichiometric constants may soon not be necessary.The solubility of calcite and aragonite in seawater was studied over a wide range of salinities (5-44 per mil) and temperatures (5-40°C) at 1 atm total pressure. The study involved measuring solubility from both undersaturated and supersaturated solutions using a closed system technique, with equilibration periods ranging from 5 days to nearly 2 years. Over 480 independent measurements were conducted, and results were compared to previous studies. The stoichiometric solubility constants for calcite and aragonite were found to be statistically equivalent to those determined by several earlier researchers. However, this study is the first long-term equilibration investigation of aragonite solubility in seawater. The study used thermodynamic solubility products of calcite and aragonite and combined them with recent methods for estimating total ion activity coefficients in complex electrolyte solutions to analyze the data. The findings indicate that the influence of coprecipitates in equilibrium or solubility-controlling phases on the solubility behavior of calcium carbonate cannot be discerned within the present precision (approximately 5%). The study highlights the importance of equilibration time in determining the solubility of aragonite in seawater, as previous investigations may have failed to reach equilibrium. The results show that the temperature coefficients for both calcite and aragonite are small and negative at all salinities. The stoichiometric solubility constants of calcite and aragonite in seawater were determined and compared to previous studies, with the results showing good agreement for calcite but significant differences for aragonite. The study also discusses the importance of ion pairing in the calculation of free ion activity coefficients and the impact of these factors on the solubility of calcium carbonate in seawater. The results indicate that the solubility of calcite and aragonite in seawater closely follows the predicted solubility of pure calcium carbonate, despite the presence of mixed carbonates in equilibrium with seawater. The study concludes that recent refinements in estimating activity coefficients in complex electrolyte solutions have reached an accuracy where the use of stoichiometric constants may soon not be necessary.