This paper describes chemical equilibrium models for predicting carbonate and silica scale formation, CO2 breakout and H2S gas exchange in geothermal brine systems to high concentration and temperature. The equilibrium description is based on a minimization of the free energy of the system with solute activities described by the semiempirical equations of Fitter (1973; 1987). The carbonate model is parameterized by appropriate osmotic, electromotive force and solubility data(T less than or equal to 250 degrees C) available in binary and ternary solutions in the seawater Na-K-H-Ca-Cl-SO4-H2O system. The silica model is parameterized by solubility data to 320 degrees C in the Na-Mg-ClSO4-SiO2-H2O system. The H2S model is parameterized by solubility data in the H2S-NaCl-H2O system to 320 degrees C. The predictive capabilities of the models are demonstrated by comparison to both laboratory and field data. Examples have been given to illustrate the use of the carbonate model to predict downhole brine compositions in contact with specified formation minerals, temperature and pressure effects on carbonate scaling, the effect of scale inhibitors and breakout characteristics. Application of the silica model demonstrates the effect of temperature on silica scale formation. These illustrations show that the models can be used to reliably predict important chemical behavior in geothermal operations.