Abstract The purpose of this chapter is to give an account of the accuracy required to characterize chemical equilibria between aqueous fluids and minerals in the marine environment, and the answer brought to this challenge by the recent improvements in the calculation of the thermodynamic properties of multicomponent aqueous electrolyte solution models, namely Pitzer’s ion interaction approach, and mineral solubility models based on this formalism. We focus on the general outline leading to the construction of a chemical model for seawater type solutions in the Na-K-Ca-Mg-Sr-Ba-Cl-SO 4 -H 2 O system, and to the solubility calculations it allows for a few minerals (barite, celestine, gypsum) as a function of solution composition, temperature and pressure. This model is then applied to the analysis of the saturation state of ocean waters and marine sediment pore waters, as examples of what can be expected from the use of such models to address the general question of chemical equilibrium between minerals and aqueous fluids in natural settings from the case of the marine environments.