We applied the internal photoemission technique to the direct observation of deep levels together with barrier heights and band discontinuities at different semiconductor heterostructure interfaces. Its performances and capabilities are superior to those of established techniques: the energy resolution is excellent and it can be applied to "real" device interfaces under bias. The technique was successfully tested for the Pd/n-GaAs and Pd/a-Si:H Schottky barriers as well as for the a-Ge/n-GaAs and a-Ge/p-GaP heterojunctions. We found a Pd/n-GaAs Schottky barrier height of 0.78 eV and an a-Ge/n-GaAs valence band discontinuity of 0.42 eV; in both cases the Fermi level was "pinned" at an averaged value of 0.76+/-0.03 eV above the top of the GaAs valence band. Furthermore we directly measured a feature at 1.09 eV, due to Pd-induced empty states approximately 1.1 eV above the valence band. The Pd/n-GaAs Schottky barrier also exhibits a feature at 1.36 eV, explained by transitions from the valence band to a localized state, made possible by the band bending. In the case of a-Ge/n-GaP we found a DELTA-E(V) = 0.88 +/- 0.05 eV and a DELTA-E(C) = 0.68 +/- 0.05 eV. Finally, in the Pd/a-Si:H Schottky barrier, we demonstrated the contribution of resonant tunneling through localized states of the conduction band.