Aluminosilicate glasses containing rare-earth and rare-earth analogue cations such as Y3+ and La3+ are interesting for a variety of technological applications, as well as for elucidating general principles of glass formation and structure. We present high-resolution O-17 and Al-17 NMR data on a series of lanthanum and yttrium aluminosilicates, which show the significant increase in concentrations 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 resolution than standard MAS and demonstrate that the latter can give misleading results for compositional effects on speciation. Oxygen sites that resemble those in crystalline Y2O3 and La2O3 were not detected, suggesting that such extreme clustering is not significant in the compositions studied, The role of highly coordinated Al, and the configurational complexity generated by the formation of such species, is discussed.