We present a semi-empirical thermodynamic model with uncertainties that encompasses the full range of compositions in H 2 O–CO 2 –NaCl mixtures in the range of 10–380°C and 1–3500bars. For binary H 2 O–CO 2 mixtures, the activity–composition model is built from solubility experiments. The parameters describing interactions between H 2 O and CO 2 are independent of the absolute thermodynamic properties of the end-members and vary strongly non-linearly with pressure and temperature. The activity of water remains higher than 0.88 in CO 2 -saturated solutions across the entire pressure–temperature range. In the H 2 O–NaCl system, it is shown that the speciation of aqueous components can be accounted for by a thermodynamic formalism where activities are described by interaction parameters varying with intensive properties such as pressure and temperature but not with concentration or ionic strength, ensuring consistency with the Gibbs–Duhem relation. The thermodynamic model reproduces solubility experiments of halite up to 650°C and 10kbar, and accounts for ion pairing of aqueous sodium and chloride ions with the use of associated and dissociated aqueous sodium chloride end-members whose relative proportions vary with salinity. In the H 2 O–CO 2 –NaCl system, an activity–composition model reproduces the salting-out effect with interactions parameters between aqueous CO 2 and the aqueous species created by halite dissolution. The proposed thermodynamic properties are compatible with the THERMOCALC database (Holland and Powell, 2011) and the equations used to retrieve the activity model in H 2 O–CO 2 can be readily applied to other systems, including minerals.