Solubility isotherms of the sparingly soluble salts CaF2(s) and CaSO4·2H2O(s) in their mixed aqueous solutions have been measured at 298.1 K. It was found that the CaF2(s) solubility decreases with increasing CaSO4 concentration in the solution and reaches about 1/3 of the CaF2(s) solubility in pure water in the CaSO4·2H2O(S) saturated solution. A thermodynamic model was developed to predict the CaF2(s) solubility isotherm in this system, in which the short range interactions of the species in the aqueous solution are represented by ion-association constants reported in literature, and the long range interaction, i.e., the electrostatic term, is represented by the well known Davies equation. The predicted solubility isotherm reasonably agrees with the experimental results. The contributions of the long-range term and the short-range term to the calculated solubility isotherm were investigated. It was concluded that the ionic association combining with the Davies equation is sufficient to represent the excess interaction of the CaF2 + CaSO4 aqueous solution at 298.15 K. This model approach could be applicable for other dilute mixed electrolyte systems in which component activity coefficients are lacking and model parameters are difficult to determine.