The structure of homogeneous binary SiO2-Al2O3 glasses with 0.4 to up to 12.0 wt % Al2O3 have been studied using high-resolution Al-27, O-17, and Si-29 NMR spectroscopic techniques. All glasses are found to contain a mixture of 4-, 5-, and 6-fold coordinated Al sites (Al-IV, Al-V, and Al-VI). The relative proportions of these sites are strongly dependent on composition with Al-IV (All) being most dominant in glasses with < 1 wt % (greater than or equal to 7 wt %) Al2O3. On the other hand, no significant dependence of Al speciation on fictive temperature is observed. The coordination polyhedra of a significant fraction of the Al-VI sites in these glasses are found to be unusually distorted, similar to that in the case of crystalline Al2SiO5 polymorphs. The O-17 NMR spectra show the presence of three types of oxygen sites, Si-O-Al-V, Si-O-Al-IV, and Si-O-Si, in these glasses. The Si-29 MAS NMR spectra corroborate these results and reveal the presence of Q(4) sites with and without Al next-nearest neighbors. It is hypothesized that the tetrahedral [AlO4/2](-1) units in glasses with < 1 wt % Al2O3 are predominantly charge balanced by the formation of oxygen triclusters, whereas the Al-V and Al-VI sites play the role of charge compensators in glasses with higher Al content. Addition of either low field strength alkali ions such as K+ or high field strength rare earth ions such as La3(+) to these glasses results in charge balance and stabilization of Al-IV sites.