GaN, AlxGa1–xN, and AlN layers exhibit interference oscillations and bandgap-related features in their reflectance-difference (-anisotropy) (RD/RA) spectra. We concentrate on the interpretation of interference-related data, providing a general expression for these optical anisotropics and discussing mechanisms that originate in the layers themselves. These include anisotropic strain in the plane of the layer, a tilt of the c axis with respect to the surface normal, and non-normal-incidence illumination. We estimate the magnitudes of these contributions, and show that they are consistent with those observed. In principle these contributions can be separated by their different azimuthal dependences. The complex pattern of the data for AlxGa1–xN and AlN indicate that contributions from several layers are present.