This article reports on a study of the ultraviolet (UV)-ozone oxidation of GaAs(100) and InP(100) using high-resolution x-ray photoelectron spectroscopy (XPS) complemented by transmission electron microscopy (TEM). For GaAs(100), various oxides As2O3, As2O5, and Ga2O3 are observed. The As2O3 core level intensity follows a logarithmic growth, while AS2O5 and Ga2O3 exhibit a linear growth as a function of UV-ozone exposure time. Angle-resolved XPS and TEM measurements confirm that the oxide has a layered structure represented sehematically as (AS2O5, Ga2O3)/(AS2O3, Ga2O3)/GaAs(100). Two different oxidation processes likely occur; the first is bulk oxidation at the GaAs(100)/oxide interface, and the second process occurring at the higher oxide/lower oxide interface is conversion of the lower As oxide to the higher As oxidation state. On InP (100), oxides corresponding to In2O3 and PO(x) are observed, and the respective photoelectron intensities follow a logarithmic growth as a function of UV-ozone exposure time. Angle-resolved XPS measurements show that the In2O3 and PO(x) are homogeneously mixed. The total oxide thickness, calculated from the photoelectron intensity ratio, follows a logarithmic growth as a function of exposure time on both GaAs(100) and InP(100).