Isothermal interdiffusion coefficients have been obtained by Gouy interferometry for the ternary system NaCl-MgCl[sub 2]-H[sub 2]O at 25.00[degrees]C and a 1:1 mole ratio of NaCl to MgCl[sub 2]. Data are reported for total molar concentrations of 0.5, 1.0, 2.0, 3.0, and 3.72 M. Diffusion coefficients at mole ratios of 1:1 (0.5 and 1.0 M), 1:3 (1.0 and 3.0 M), and 3:1 (1.0 M) were also obtained by Rayleigh interferometry during the corresponding Gouy experiments by switching the two optical systems back and forth. As expected, good agreement is found between Gouy and Rayleigh results. Diffusion coefficients at the 1:1 ratio are intermediate between the results for 1:3 and 3:1 ratios, with D[sub 12] becoming large with increasing concentration but always less than D[sub 11] and D[sub 22], and with D[sub 11] and D[sub 22] crossing at 2.8 M. Densities were measured for all solutions. Rayleigh experimental and data analysis techniques are described. 91 refs., 8 figs., 7 tabs.