This paper compares the Auger and ESCA techniques used to characterize the interfaces of thermally grown and anodic oxides of InP and GaAs. In the anodic oxide of GaAs, the Ga–O bonding extends deeper into the GaAs than the As–O bonding. In the anodic oxide of InP, both the P–O and In–O bonding penetrate to the same depth. The anodic oxide–GaAs interface changes with electrolyte. There is elemental P at the interface of the thermal oxide on InP, but it was not possible to prove or disprove the existence of elemental As at the anodic oxide/GaAs interface. It was found that the ESCA technique provided much needed bonding information which greatly facilitates characterizing the interface. It appears essential that both composition and bonding be determined.