Density and viscosity of 0.25[xKSCN + (1 - x)NaSCN] + 0.75CH(3)CONH(2) melt systems were measured as functions of temperature (293.15 less than or equal to T/K less than or equal to 343.15) and mole fraction, x = 0.0 to 1.0. The temperature dependence of viscosity has been analyzed by using the Vogel-Tammann-Fulcher (VTF) equation. Molar volumes and intrinsic volume were found to be additive in nature. The existence of the mixed alkali effect (MAE) has been established by the significant negative deviation from the additivity in the ideal glass transition temperature and in the viscosity isotherms. The onset of the MAE in the present system has been explained by the mutual contribution of the polarization effect of SCN- ion on the Na+ ion and the presence of the rigid polymeric type structure, which decrease as the Na+ ions are progressively replaced by the K+ ions.