We report measurements of the conduction‐band offset in GaAs‐Ga1−xAlxAs multiquantum wells. Admittance spectroscopy is used to measure the activation energy for thermionic emission of electrons over GaAlAs barriers and we discuss the validity of the technique in the presence of a low electric field. We show that the usual approximation in calculating the position of the Fermi level at 0 K can lead to significant error in the determination of the band offset. For an alloy compositionx=0.39, we found a conduction‐band offset of 303 meV.