The density and the viscosity of aqueous (NH 3 + CO 2 ) solutions have been measured at atmospheric pressure over a temperature range of 278 to 318 K and apparent concentrations of solute between 4 and 10 mol NH 3 kg H 2 O −1 for NH 3 and between 1 and 5.2 mol CO 2 kg H 2 O −1 for CO 2 . Solute loss from the aqueous solution associated with the high equilibrium partial pressure of NH 3 and of CO 2 has been limited by devices and protocols developed adhoc for sample preparation and density and viscosity measurement. Solid or vapor formation out of the initial liquid mixture has been avoided by means of a thermodynamic model-driven design of experiments. The experimental values gathered in this work have been used to obtain empirical models of density and viscosity, as a function of the apparent concentration of solutes, that is, NH 3 and CO 2 , and temperature, that are able to reproduce the experimental values in all cases with deviations below 1.1% for density and below 9.4% for viscosity. While it is the first time that, to our knowledge, experimental viscosity data of aqueous ammonia solutions loaded with CO 2 are added to the literature, the density model developed in this work is also able to reproduce experimental density data from literature with deviations below 1.5%, even outside the boundaries of the experimental conditions considered in this work.