The (p , ρ , T , x ) properties for {x NH3 + (1 − x )H2O} with x = (0.1048, 0.2046, 0.3807, 0.5565, 0.7008, 0.8010, 0.9102, and 1.0000) in the compressed liquid phase were measured by means of a metal-bellows variable volumometer at temperatures of (450 and 500) K and pressures from (10 to 200) MPa. The volume fraction purities of the ammonia used in the measurements were 0.99999. The water employed was deionised, distilled, and degassed to 3.0 ≅ 10−4 Pa by freeze–thaw cycling with liquid nitrogen before use. In order to carry out these measurements at higher ranges of temperature (above T = 450 K) and pressure (up to p = 200 MPa), we have implemented several reconstructions to the apparatus, including the replacement of the sample cell containing the metal-bellows, and the development of a thermostatted air bath, in which the temperature of the air and sample cell can be raised up to 600 K. The expanded uncertainties (k = 2) in temperature, pressure, density, and composition measurements have been estimated to be less than 5 mK; 1.3 × 10−2%; 0.48%; and 8.2 × 10−4, respectively. Comparisons of the available equations of state (EoS) with the present measurements are reported. On the basis of the present results, moreover, the excess molar volume vmE of the mixtures was calculated and illustrated as a function of temperature and pressure.