Contactless electroreflectance (CER) spectroscopy has been applied to study band bending in Ga In N As Sb / Ga As quantum well (QW) structures. It has been observed that CER features significantly changes upon annealing: the period of GaAs-related Franz-Keldysh oscillations increases; intensities of excited QW transitions rose compared to the intensity of the fundamental QW transition. The observed changes in CER spectra have been explained by a shift of the Fermi level in the GaInNAsSb layer: the defect states in as-grown GaInNAsSb tend to pin the Fermi level at an energy characteristic for these defects; annealing removes defects from this material and effectively shifts the Fermi level to the conduction band.