Infrared reflectance spectra of two strategically selected series of glasses in the Na2O-Al2O3-SiO2 system have been measured. One series is along the SiO2-NaAlO2 join with NBO/T = 0. The other series is on the Na2O-SiO2 binary with NBO/T from 0.67 to 1.33. Kramers-Kronig analysis was performed, and the far-infrared region of the computed dielectric loss function is interpreted in terms of changes in the Na structural environment. On the basis of the strong, positive, linear correlation between peak height and Na content, the observed far-infrared peak is assigned to vibration of the Na ions. The Na mode occurs at 85 cm−1 in the spectra of all the fully polymerized glasses. The vibrational energy increases with depolymerization, to 150 cm−1 for the most depolymerized glass, owing to the gradual collapse of the tetrahedral framework structure. Calculated Na-O force constants range from 10 N/m for the fully polymerized glasses to 30 N/m for the most depolymerized composition. The results of this study agree with a structural model, for glasses with Na/Al ≥ 1.0, in which Na charge-balances tetrahedral Al, with excess Na acting as network modifiers and depolymerizing the structure.