The infrared absorption spectra of sodium-disilicate glasses containing various amounts of Fe O ([Na O · 2 SiO ] [Fe O ] , where x = 0.05, 0.1 and 0.2) were investigated in the wavenumber range from 200-2000 cm . The addition of Fe O to the sodium-disilicate glass does not seem to introduce any new absorption band as compared with the spectrum of a pure sodium-disilicate glass; nevertheless, a general shift of the existing absorption bands toward lower wavenumbers is observed. The amount of shift is, in fact, proportional to the content of Fe O in the glass. This observation is consistent with the recently proposed structural model for the bonding of Fe ions in the iron-sodium-silicate glass system. Annealing of 20 mol% iron oxide glasses at 550 and 580°C produced an extra sharp infrared absorption peak at about 610 cm wavenumber. This new peak is believed to be related to the crystallized particles of the glass as concluded from both a scanning electron micrograph and an electron diffraction pattern. © 1980.