The quaternary III–V compound semiconductor GaInAsP is an important material for many optoelectronic devices, the surface of which generally needs to be passivated in the fabrication of such devices. Thus understanding the surface chemistry and monitoring the surface band gap states after oxidation and sulphur passivation have become necessary. Further, understanding the effect of ion bombardment on the GaInAsP surface during dielectric deposition is also of importance for device fabrication. In this study, quaternary GaInAsP(100) surfaces were subjected to UV/ozone and wet chemical treatments, dilute HF etching, sulfur passivation, and Ar ion bombardment. The composition and the relative movement of the surface Fermi level(EFs)of the surfaces were measured by x-ray photoemission spectroscopy (XPS) after oxidation, HF etching, sulfur passivation, and ion bombardment of surfaces. It was found that oxidation by ozone exposure formed multiple oxide phases of all the constituent elements. Both HF etching and sulfur passivation treatments were effective in generating surfaces having almost no oxide. It was also found that while sulfur passivation combined with an ultrahigh vacuum annealing at 300 °C reduced the surface band bending onn-type GaInAsP(100), it invertedp-type GaInAsP ton-type. AnL-edge absorption spectrum of the sulfur passivated surface confirmed the presence of a sulfur layer. Further, it was found that an Ar+ion bombardment pins theEFsnear the midgap for bothn- andp-type GaInAsP surfaces. ©1997 American Institute of Physics.