Low energy optical modes of molecular beam epitaxy-grown In1-x Gax N thin films with 0≤x≤0.6 are investigated using infrared reflectance measurements. We found that the reflectance of the films for wave vectors in the range from 600 to 800 cm-1 is determined by the high energy E1 (LO) -plasmon coupled modes. In the higher energy regime of the UV-visible reflectance spectrum of InN, critical points with energies 4.75, 5.36, and 6.12 eV belonging to A and B structures are observed. The energies of these critical points increase with increasing values of x, similar to the band gap energy of these films. © 2010 American Institute of Physics.