We report 17 O magic-angle spinning (MAS) NMR data for crystalline NaAlO 2 and CaAl 2 O 4 at external magnetic fields of 9.4 and 14.1 T, as model compounds for Al-O-Al sites in tetrahedral networks. The former contains one peak with isotropic chemical shift (delta iso ) = 30.9 ppm and quadrupolar coupling constant (C Q ) nearly equal 1.8 MHz. The latter contains several peaks with delta iso ranging from 39 to 87 ppm and C Q nearly equal 1.5 to 2.4 MHz. Triple-quantum MAS (3QMAS) spectra of sodium and calcium aluminosilicate glasses with Si/Al<1 show clearly resolved peaks for Al-O-Al sites, removing ambiguity in the use of such spectra to explore the extent of aluminum avoidance in both glassy and crystalline materials. We also report 23 Na and 27 Al NMR data for the crystalline phases.