The viscosities of the four series of mixed alkali glass melts such as 25(Na₂O+K₂O)⋅75SiO₂, 25(Na₂O+K₂O)⋅5Al₂O₃⋅70SiO₂, 25(Na₂O+K₂O)⋅7.5Al₂O₃⋅67.5SiO₂ and 25(Na₂O+K₂O)⋅10Al₂O₃⋅65SiO₂ as well as their T_g were measured to elucidate a relation between property behaviors and mixed alkali effect models. The base mixed alkali glass melts (25R₂O⋅75SiO₂) showed a mixed alkali effect, namely viscosity minima at the composition K₂O/R₂O≈0.25, which become weaker as temperature increases. The activation energy for the viscous flow of this base glass melts showed also a minimum at this composition. However, the mixed alkali effect disappeared, as SiO₂ is substituted by Al₂O₃. This implies that the nonlinearity disappears with decreasing nonbridging oxygen content. In the case of T_g no clear tendency with respect to the Al₂O₃ content was found.
By comparison of the present work with the earlier work of resistivity, two distinct differences between the mixed alkali effects in viscosity and electrical conductivity were found. From these results, it was concluded that the mechanism for the mixed alkali effect in both properties should be treated independently.