Abstract: Activity coefficients of NaCl in water N,N dimethylformamide (DMF) mixed solvents are determined at 298.15K by EMF measurement with sodium ion selective electrode and Ag AgCl electrode. It is shown that the activity coefficient of NaCl decrease with the increase of concentration of DMF in mixed solvents. The viscosities of water DMF NaCl ternary system at different temperature have also been measured. The experimental viscosities have been correlated by using pseudo binary solute aggregation model presented by Liu et al. The mean deviation between calculated results and experimental data is 2.5% .