In many natural and industrial processes the thermodynamic properties of aqueous electrolyte solutions play an important role. Systems of interest are frequently characterized by a large number of components and possible reactions. Due to low solubility, some reaction products precipitate as solids. The thermodynamic modelling therefore implies the computation of simultaneous phase and chemical equilibria. The minimization of the Gibbs free energy and the K-value method are powerful algorithms for numerical computation and permit the simulation of complex electrolyte systems. The behaviour of the liquid solution has to be described by suitable activity coefficient models.