Optical measurements (spectrophotometry in the range 0.2-25 mu m and spectroscopic ellipsometry in the range 0.35-0.9 mu m) have been performed on AIN thin films of varying thicknesses (0.07-0.7 mu m) using two different sputtering methods. Since aluminium nitride has a semi-conducting behaviour, absorption, due to direct and indirect transitions, it is observed in the UV range and the band gap can thus be determined. In the visible range; the absorption is very low and the Sellmeier formula provides a correct description of the real part of the complex index. In the middle infrared range, optical properties are dominated by phonon behaviour, and the measurements an well described by a two oscillator simulation.