Modified van Hoof structures containing GaInNAs(Sb) quantum wells (QWs) with indium content varying from 8% to 32% have been investigated using contactless electroreflectance (CER) spectroscopy. In CER spectra, both the QW transitions and GaAs-related Franz–Keldysh oscillations (FKOs) have been clearly observed. The band gap discontinuity at the GaInNAs(Sb)/GaAs interface has been determined by analyzing the QW transitions. The built-in electric field in the GaAs cap layer has been extracted from the FKO periodicity. The Fermi level position in the GaInNAs(Sb) QW has been determined through knowledge of the electric field in the GaAs cap layer and band gap discontinuity in the GaInNAs(Sb)/GaAs QW. It has been found that the Fermi level is pinned for all samples at the same energy, ∼4.7 eV below the vacuum level. The Fermi level is located very close to the Fermi level stabilization energy, ∼4.9 eV below the vacuum level. A high concentration of native point defects in the as-grown material is the reason for the Fermi level pinning at this energy.