We report the densities of SrCl2(aq), Na2SO4(aq), NaHCO3(aq), and the synthetic reservoir brine system [x(1)NaCl + x(2)KCl + x(3)MgCl(2) + x(4)CaCl(2) + x(5)SrCl(2) + x(6)Na(2)SO(4) + x(7)NaHCO(3)](aq), where x denotes the mole fraction of the different salts, at temperatures between (298 and 473) K and pressures up to 68.5 MPa. The molalities at which the single-salt solutions were studied are (1.022, 2.024, and 3.031) mol.kg(-1) for SrCl2(aq), (0.783 and 1.502) mol.kg(-1) for Na2SO4(aq), and (0.507 and 1.000) mol.kg(-1) for NaHCO3(aq). The compositions of the synthetic reservoir brines studied are, first, x(1) = 0.770, x(2) = 0.022, x(3) = 0.040, x(4) = 0.105, x(5) = 0.002, x(6) = 0.044, and x(7) = 0.018 with molality 0.359 mol.kg(-1) and, second, x(1) = 0.797, x(2) = 0.006, x(3) = 0.042, x(4) = 0.145, x(5) = 0.002, x(6) = 0.006, and x(7) = 0.002 with molality 1.900 mol.kg(-1). The measurements were performed with a vibrating-tube densimeter calibrated under vacuum and with pure water over the full ranges of pressure and temperature investigated. An analysis of uncertainties shows that the expanded relative uncertainty of density with a coverage factor of 2 is bounded by a linear function of molality b increasing from 0.03 % at b = 0 mol.kg(-1) to 0.06% at b = 3 mol.kg(-1). An empirical correlation is reported that represents the density for each brine system as a function of temperature, pressure, and molality with absolute average relative deviations of approximately 0.02 %. Comparing the model with the available database of literature results, we find absolute average relative deviations of 0.10 %, 0.03 %, and 0.01 % for the systems SrCl2(aq), Na2SO4(aq), and NaHCO3(aq), respectively. The model can be used to calculate the density, apparent molar volume, and isothermal compressibility over the full ranges of temperature, pressure, and molality studied in this work. An ideal mixing rule for the density of a mixed electrolyte solution was tested against our synthetic reservoir brine data and was found to offer very good predictions at all conditions studied with an absolute average relative deviation of 0.05 %.