The viscosity of peraluminous Na 2 O-Al 2 O 3 -SiO 2 melts of constant SiO 2 content is essentially independent of Al 2 O 3 content. The addition of more Al 3 + to the peraluminous melts does not result in a decrease in viscosity. This finding indicates that the Al 3 + 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 Al 3 + 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 SiO 2 contents shows that the viscosity of melts in the Na 2 O-Al 2 O 3 -SiO 2 system form the same trend ′1 log 1 0 unit as a function of Na:Al ratio within the 10 8 -10 1 4 Pa's range.