A formula for the calculation of the refractive index of AlXGa1-XN in the infrared region has previously been proposed. In this paper, the validity of the proposed model is assessed against the multi-oscillator model which simulates the infrared reflectance of AlXGa1-XN epilayers. Acceptable agreement was found between the two models for wavenumbers greater than 3500 cm(-1). Additional validation for the earlier proposed formula is obtained by the calculation of epilayer thicknesses of AlXGa1-XN samples in the infrared region.