The optical properties of hexagonal AlxGa1−xN x from 0.05 to 0.42 epitaxial films with Sim doping concentrations up to 1018 cm−3 grown on c-plane sapphire substrates by metal-organic chemical vapor deposition have been investigated using infrared reflectance spectra. The dielectric functions=1+i2 of the AlxGa1−xN films are determined in the wavelength region of 1.54-50m at room temperature. The experimental reflectance spectra are analyzed using classical harmonic Lorentz oscillators and Drude model in the transparent,reststrahlen, and free carrier absorption regions. GaN-like E1 transverse-optical TO phonon frequency linearly decreases and its strength increases with decreasing Al composition. However, AlN-like E1 TO phonon frequency shows a relatively weak composition dependence and its strength increases with increasing composition. At a wavelength of 1.54m, 1 varies between 4.86 and 5.2 when the composition changes from 0.05 to 0.42 and 2 is close to zero. The longitudinal-optical phonon plasmon LPP coupled modes of n-type hexagonal AlxGa1−xN films are also discussed. For samples with higher concentrations beyond 1018cm−3 the upper LPP coupled mode frequencies increase with increasing carrier concentration indicating the transition from phononlike to plasmonlike behavior.