This work is on in the volumetric and thermal properties of brines in the quinary Na–K–Ca–Mg–Cl–H2O chemical system. Its objective is twofold. First, by acquiring original data for temperatures ranging from 278.15 to 353.15 K and ionic strengths ranging from 1.3 to 5.1 mol·kg–1 it aimed to add to the experimental data set, usually acquired only at high ionic strengths or at 298.15 K. Experimental solution density was measured using a vibrating-tube densitometer with relative uncertainty, Δρ/ρ, better than 6 × 10–6. This property, combined with volumetric heat capacity measurements, provided the isobaric heat capacity of solution determined with a mean relative deviation better than 0.3%. Second, we used PhreeSCALE software to compute the density and heat capacity of the chemical system of interest, simultaneously applying the Pitzer and the Helgeson–Kirkham–Flowers (HKF) equations. We propose a new set of specific interaction parameters so that published and newly measured experimental data can be described accurately. We show that only binary interaction parameters are necessary and that ternary interaction parameters could be set to zero.