The IR spectra of thin amorphous films of mixed oxides In 2 O 3– SnO 2 system have been studied in the spectral range 4000–400 cm-1 by Fourier transform infrared technique. The effects of changes in composition, film thickness, substrate temperature and annealing on the IR absorption bands are observed. A shift in frequency and intensity of the bands with varying preparation parameters and an increase in ordering at higher annealing temperatures are observed in the infrared curves. Some new peaks appear, which reveal the presence of lower valency states both in In 2 O 3 and SnO 2. The disappearance of bands at higher annealing temperatures is assigned to the removal of point defects in which oxygen plays an important role. The shift in band frequency with an increase in (i) the Sn content is attributed to the incorporation of Sn 4+ ions in the In 2 O 3 lattice, (ii) the film thickness is attributed to the large concentration of donor centers, (iii) the substrate temperature is attributed to the increase in diffusion of Sn atoms from interstitial locations into the In cation sites, and (iv) the annealing temperature is attributed to the oxygen vacancies and indium interstitials.