The viscosity of peraluminous Na2O-Al2O3-SiO2 melts of constant SiO2 content is essentially independent of Al2O3 content. The addition of more Al3+ to the peraluminous melts does not result in a decrease in viscosity. This finding indicates that the Al3+ in these melts does not enter the structure as a viscosity reducing network-modifier. The most probable charge-balanced structural unit in these melts is a tri-cluster that involves one Al-tetrahedron sharing an O atom with two Si-tetrahedra. Both viscosity and activation energy for viscous flow in the investigated viscosity range in these peraluminous melts are largely unaffected by the Al3+ content, indicating that increasing the proportion of tri-clusters does not significantly affect the mechanism of viscous flow. Comparison of data for melts of different SiO2 contents shows that the viscosity of melts in the Na2O-Al2O3-SiO2 system form the same trend ±1 log10 unit as a function of Na:Al ratio within the 108–1014 Pa·s range.