The energy barriers forp‐nGe&sngbnd;AlxGa1−xAs and GaAs&sngbnd;AlxGa1−xAs heterojunctions have been determined by capacitance measurements. The results indicate that the valence band of AlxGa1−xAs tends to remain fixed with respect to the vacuum level for different values ofx. This leads to spiked barriers in the conduction band for these junctions. The values determined for the barriers are consistent with measurements onp‐pGe&sngbnd;GaAs junctions.