Specific heat capacities at constant volume (c(v)) of {xNH(3) + (1 - x)H2O} (x approximate to 0.7, 0.8, 0.9) mixtures were measured with an adiabatic calorimeter. Temperatures ranged from 300 K to 520 K, and pressures ranged from 3 MPa to 20 MPa. Measurements were conducted on single-phase liquid and compressed gaseous samples. The mixtures were gravimetrically prepared from high-purity substances and verified by chemical analysis. Density was reported for initial and final end points during each calorimetric experiment. The principal sources of uncertainty are the temperature rise measurement and the change-of-volume work adjustment. The expanded relative uncertainty (with a coverage factor k = 2 and thus a 2-standard-deviation estimate) for c(v) is estimated to be 1% for liquid phase and 4% for gaseous results, and for density it is 0.2%.