The structures of roller‐quenched SiO2‐Al2O3glasses containing 10 to 50 wt% Al2O3were investigated by TEM and high‐resolution27Al and29Si magic angle sample spinning nuclear magnetic resonance (MASS‐NMR) spectroscopy. The chemical shifts observed in the NMR spectra of these phase‐separated (on the TEM scale) glasses provide evidence, for the existence of all three (4‐, 5‐, and 6‐jold) Al‐coordination units in these glasses and for the presence of A1 in both the Si‐rich