Amorphous indium nitride (a-InN) thin films were deposited onto different substrates at temperatures <325K using RF magnetron sputtering at a rate 0.3–0.4Å/s. X-ray diffraction patterns reveal that the films grown on the substrates are amorphous. The optical absorption edge, ‘bandgap’ energy, E g , of a-InN has been determined by spectroscopic ellipsometry over the energy range 0.88–4.1eV. The absorption coefficient was obtained by the analysis of the measured ellipsometric spectra with the Tauc–Lorentz model. The E g was determined using the modified Tauc and Cody extrapolations. The corresponding Tauc and Cody optical bandgaps were found to be 1.75 and 1.72eV, respectively. These values are in excellent agreement with the values of the bandgap energy obtained as fitting parameters in the Tauc–Lorentz model: 1.72±0.006eV as well as by using spectrophotometry (1.74eV) and photoluminescence (1.6eV). The spectral dependence of the polarized absorptivities was also investigated. We found that there was a higher absorptivity for wavelengths <725nm. This wavelength, ∼725nm, therefore indicates that the absorption edge for a-InN is about 1.70eV. Thus, the average value of the measured optical absorption of a-InN film is approximately 1.68±0.071eV.