An infrared reflection technique has been developed for the characterization of GaN and GaN/AlGaN epitaxial layers and multilayers. The infrared light is brought to the III-nitride surface with a KRS-5 internal reflection crystal. The technique is complimentary to Raman measurements and provides information on the longitudinal and transverse optical (LO) modes of GaN and ALN. For thin GaN layers, the modes of the thin, 22-nm-thick A1N nucleation layer can be clearly observed in the spectra. The free carrier concentration of the GaN can be characterized by analyzing the LO phonon-plasmon coupled mode present in doped samples. Because the light is multiply reflected in the GaN layer, the technique can potentially detect impurities (such as H or C) in the GaN. The application of this technique to characterize high quality molecular beam epitaxial layers is discussed.