An infrared spectroscopic technique has been developed for the characterization of GaN epitaxially grown thin films on sapphire substrates. The technique is complementary to Raman measurements, and provides information on the longitudinal (LO) and transverse optical modes of GaN, and, for thin GaN films, the sapphire substrate. In addition, the free carrier concentration of the GaN can be characterized by analyzing the LO phonon-plasmon coupled mode present in doped samples. For lightly doped films, the GaN film thickness is easily obtained from the Fabry-Perot fringes present in the spectra.