The pseudodielectric function spectra 〈ε〉 of wurtzite GaN, AlGaN, and GaN/AlGaN heterostructures were determined for photon energies ranging from 2 to 5 eV, using variable angle spectroscopic ellipsometry (SE). Samples were grown by low-pressure metalorganic chemical vapor deposition on c-plane sapphire substrates. The experimental 〈ε〉 spectra were analyzed using a multilayer approach, describing the dielectric functions of the individual layers by a parametric oscillator model. In this way, parametric dielectric function spectra of GaN and AlxGa1−xN (x⩽0.16) were derived, as well as the composition dependence of the AlxGa1−xN band gap energy. The SE band gap data were found to be consistent with a bowing parameter close to 1 eV. Finally, the GaN and AlxGa1−xN parametric dielectric functions were used to quantitatively analyze the pseudodielectric function spectrum of GaN/AlGaN modulation doped field effect transistor structures, demonstrating the potential of SE in combination with a multilayer parametric dielectric function model for nondestructive ex situ control of GaN/AlGaN device structures.