The isopiestic osmotic coefficients, densities, and speeds of sound of aqueous mixtures of NaCl with guanidinium chloride (GnCl) have been measured at different ionic strengths with varying compositions at 298.15 K with a view to determine the ionic interactions. The excess free energy, Delta (m)G(E), volume, Delta V-m(E) and compressibility of mixing, Delta K-m(E), Of the NaCl-GnCl mixtures at constant ionic strength show interesting features with changing ionic strength fractions of the electrolytes. These excess properties of mixing with both negative and positive signs can be attributed to the mixing of hydrophilic and hydrophobic ions. It is shown that the binary and ternary interaction terms play an important role in the accurate representation of the osmotic coefficients, activity coefficients, volumes, and compressibilities of the mixtures. Measurements on the surface tension and viscosity of the NaCl-GnCl mixtures have also been reported. A simple equation incorporating like charge interactions has been employed to correlate the surface tension of the mixtures. The mixture viscosities have also been estimated from the parameters of pure NaCl and GnCl with an empirical mixing rule.