An experimental apparatus is presented for the measurement of thermodynamic equilibria in aqueous suspensions. The solubility of calcite and gypsum, measured at 298.15 K in aqueous solutions with calcium chloride and organic acids as formic, acetic and adipic acid, is compared to data calculated using Pitzer's electrolyte activity coefficient model with the parameterisation proposed by Rosenblatt. In all investigated systems, the description of solid solubility is poor at higher ionic strength. To extend the range of application new Pitzer parameters for the Ca+/HCO3- and Ca2+/SO42- interactions have been adjusted, and in case of the Ca2+/HCOO-, Ca2+/CH3COO- and Ca2+/Ad(2-) interactions Pitzer parameters were determined for the first time. Additionally, the standard Gibbs free energies of formation of calcium formate, calcium acetate and calcium adipate have been determined. With the extended model the prediction of calcite and gypsum solubility is very good up to ionic strengths of approximately 6 mol/kg. (C) 2004 Elsevier B.V. All rights reserved.