The band discontinuity has been determined for a GaAs/AlGaAs heterojunction prepared by molecular beam epitaxy. The conduction band‐discontinuity &Dgr;Ecand the valence‐band discontinuity &Dgr;Evwere independently obtained by theC‐Vprofiling technique, taking into account a correction for the interface charge density. The simulation was employed to confirm the reliability of the obtained band discontinuity. The &Dgr;Ecdependence on both the Al composition of the AlGaAs layer and the heterojunction structure (AlGaAs on GaAs, or GaAs on AlGaAs) was examined. We found that &Dgr;Ecand &Dgr;Evwere determined to be 62 and 38% of the band‐gap discontinuity &Dgr;Eg, being independent of the structure.