The densities of KCl-NaCl aqueous mixtures were determined at temperatures from (298.15 to 523.15) K, pressures up to 40 MPa, and over a range of compositions at ionic strengths from (0.1 to 5.8) mol.kg(-1). A vibrating-tube densimeter used for the experimental measurements provides a the accuracy on density better than 2.10(-4) g.cm(-3). The mean apparent molar volumes of the mixed electrolytes, V-mean(phi), were calculated from the experimental data. At 523.15 K, the change in V-mean(phi) from saturated vapor pressure to 40 MPa is comparable to the change with temperature at 10 MPa from ambient conditions to 523.15 K. A Pitzer-type equation was fit to the data set and used to evaluate the individual partial molar volumes of NaCl and KCl in the solutions. The volume of mixing computed from the equation is always positive and displays a nearly symmetrical increase between the NaCl and KCl end-members. It also increases with increasing salt concentration and pressure, and decreases with increasing temperature. The results of this study permit a quantitative modeling of the properties of complex aqueous solutions and simulation of fluid-rock interaction processes occurring in geothermal and hydrothermal systems.