The densities of aqueous electrolyte mixtures of NaCl + BaCl2 have been measured from 25 to 140 degrees C at constant total ionic strengths of 0.5, 1.0, 2.5, and 4.5 mol kg(-1). The mean apparent molal volumes calculated from the experimental density values have been treated using the Fitter equations, and the ion interaction parameters of mixing have been evaluated. The required apparent molal volumes for the pure aqueous BaCl2 and NaCl have been obtained from our earlier work and that of Rogers and Fitter, respectively.