We have observed direct optical transitions from deep levels (DLs) induced by misfit dislocations at buried In1−xGaxAs/GaAs heterojunction interfaces by means of photoluminescence (PL) and low-energy cathodoluminescence (CL) spectroscopies. Misfit dislocations such as these have been shown to be electrically active by previous authors. By utilizing a combination of laser wavelengths and electron excitation energies, we are able to produce a PL/CL ‘‘depth profile’’ through the buried interface region, with the relative exponentially attenuated sampling depth determined by comparison of the near band-gap emission from the In1−xGaxAs layer with that from the underlying GaAs layer. For partially relaxed (1000 Å thick) In0.08Ga0.92As films, we observe DLs at approximately 0.83, 0.96, and 1.22 eV photon energies. The 0.83 eV feature is not present in the emission from either pseudomorphic (strained, 200 Å thick) In0.08Ga0.92As films or the GaAs buffer layer and substrate, and thus is likely directly related to the introduction of dislocations. These DL features are most intense for photon/electron excitations corresponding to sampling volumes which include the interface region, implying that the mechanisms generating them are localized near the interface. This ‘‘fingerprint’’ of interfacial misfit dislocations represents a potentially valuable new tool for growth diagnostics as a direct, nondestructive probe of epitaxial and interface quality.