It is shown that band offsets in semiconductors can be studied by internal photoemission experiments. Onsets in the photocurrent are observed for photon energies exceeding the fundamental energy gaps of GaAs and Al x ga 1 − x As. The sign of the photocurrent can be related to the signs of the internal electric fields. An additional photocurrent observed at energies between the two gap energies is related to excitations from the GaAs valence band to the Al x ga 1 − x As conduction band. Extrapolation of the onset energies to flat-band condition leads to ΔE c ΔE g = 0.8 for x = 0.2.