Viscosity and thermal expansion have been measured for 12 series of lithium aluminosilicate glasses. The compositional dependency of the viscosity, glass‐transformation temperature, dilatometric‐softening temperature, and refractive index all exhibit an abrupt change in slope at an aluminum‐to‐lithium ratio between 1.05 and 1.10. The thermal‐expansion coefficients of these glasses were found to depend only on the lithium oxide content, i.e., to be independent of the aluminum‐to‐silicon ratio. These results suggest that the traditional structural model for these glasses must be modified slightly. A new model is proposed which allows some aluminum to act as a modifier in all alkali aluminosilicate glasses.