Capacitance‐voltage measurements have been simulated to find the conduction‐band discontinuity in ann‐type GaAs‐Al0.3Ga0.7As heterojunction. In the calculations the partial ionization of the donor level in the (Al,Ga)As is taken into account. It is found that exact knowledge of the donor depth is a prerequisite in order to infer the conduction‐band discontinuity from capacitance‐voltage measurements. For a dopant concentration of 3×1016cm−3on either side of the heterojunction the same apparent carrier profile is obtained with a conduction‐band discontinuity equaling 65% of the total band‐gap difference and zero donor depth as with 75% discontinuity and 100 meV donor depth. It is also shown that compensation influences the apparent carrier profile. Taking literature values for the donor depth in Al0.3Ga0.7As the discontinuity of the conduction band is estimated to be 70% of the total band‐gap difference.