Capacitance‐voltage, deep level transient spectroscopy (DLTS), and admittance spectroscopy measurements have been performed to characterize a GaAs quantum well confined either by Ga0.57Al0.43As layers or by two GaAs‐Ga0.54Al0.46As superlattices. Due to a large capacitance decrease at low temperature, the well response cannot be obtained from DLTS measurements. This capacitance step is related to the thermionic emission from the well to the barrier regions. For the case of the enlarged well in the superlattice, the electron emission takes place towards the conduction miniband. In order to deduce the conduction‐band discontinuity, admittance spectroscopy measurements have been applied. The bottom of the superlattice miniband is at 107±10 meV from the GaAs conduction‐band minimum. The corresponding band offset deduced from a theoretical calculation is &Dgr;Ec=410±10 meV=(0.65±0.02) &Dgr;Eg. For the GaAs quantum well in GaAlAs, we measure a conduction‐band discontinuity of 0.35 eV between GaAs and GaAlAs that has its minimum at theXpoint. At the &Ggr; point, the band offset ratio &Dgr;Ec/&Dgr;Egis once more confirmed.