Nitride-based heterostructures with InGaN/GaN multiple quantum wells in active region were investigated by electroreflectance spectroscopy. Two spectral lines with the stable difference between their phase parameters and slightly different energies were observed in the electroreflectance spectra under various bias voltages. Using models taking into account interference effects it was shown that electroreflectance signal at low amplitude of modulation voltage originates from the first and the last quantum wells in active region. The difference between built-in electric field strengths in these two quantum wells were estimated as 270-320 kV/cm. (C) 2017 Elsevier Ltd. All rights reserved.