The surface and interface electronic structure of mismatched InxAl1−xAs epitaxial layers grown by molecular beam epitaxy on InP have been investigated using surface photovoltage spectroscopy. The crystalline structure of the epilayers was also examined by double‐crystal x‐ray diffraction. For coherently strained layers, only a few gap states are identified. Highly strained samples with inferior crystalline quality, as judged by a broadened x‐ray diffraction peak and the absence of Pendellosung fringes, display a rich spectrum of states in the band gap. Some of the states are close to the surface while others are located in the vicinity of the InAlAs/InP heterointerfaces. ©1995 American Institute of Physics.