Aluminosilicate glasses contg. rare-earth and rare-earth analog cations such as Y3+ and La3+ are interesting for a variety of technol. applications, as well as for elucidating general principles of glass formation and structure. The authors present high-resoln. 17O and 27Al NMR data on lanthanum and yttrium aluminosilicates, which show the significant increase in concns. of AlO5 and AlO6 groups caused by the smaller radii and higher field strengths of the modifier cation in the latter. Triple-quantum magic-angle spinning (3QMAS) spectra for both nuclides show much better resoln. than std. MAS and demonstrate that the latter can give misleading results for compositional effects on speciation. Oxygen sites that resemble those in cryst. Y2O3 and La2O3 were not detected, suggesting that such extreme clustering is not significant in the compns. studied. The role of highly coordinated Al, and the configurational complexity generated by the formation of such species, is discussed.