The surface and interface electronic structure of mismatched In xAl1-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.