Chemical models implemented in PHREEQC and Reaktoro were validated against a wide range of solubility measurements of individual and binary gas systems in water and aqueous solutions containing high electrolyte concentrations at a wide range of temperature and pressure conditions. When using the same database and thermodynamic models for describing the aqueous and gaseous phases, PHREEQC and Reaktoro yield analogous results. The performance of the models for describing the equilibria between multicomponent gas systems and complex brines was assessed against the bubble point pressures reported at several geothermal sites. The models satisfactorily predict the degassing pressure of different geothermal fluids but are overall susceptible to the amount of information available (or the lack of it) in the experimental sampling reports. From the two databases tested in this work, i.e., phreeqc.dat and pitzer.dat, the former was generally more versatile for the wide range of simulated conditions. The overall good agreement between the models and the experimental data shows that PHREEQC and Reaktoro are valuable tools that can be used in the early design of geothermal facilities, conditioning of gas and liquid streams (e.g., CO2, H2, produced water) or any other systems involving interactions between water/brine, gases, and minerals.