An investigation of the chemical composition of wet thermal oxides grown on In0.5Ga0.5P and In0.5Al0.5P is reported. The oxides were grown in the temperature range 500-650 degreesC. An estimate of the expected oxide composition was obtained by the construction of three-dimensional phase diagrams of the In-Ga-P-O and In-Al-P-O systems. These diagrams indicate that under thermodynamic equilibrium, the oxide layers should be composed primarily of mixtures of InPO4 and GaPO4 on In0.5Ga0.5P and InPO4 and AlPO4 on In0.5Al0.5P. X-ray photoemission spectroscopy (XPS) sputter profiles were used to determine the distribution of elements in the oxide layers and to identify the chemical compounds. The binding energy shifts observed in the XPS data suggests that the oxides are composed primarily of metal phosphates with low concentrations of the metal trioxides. At lower growth temperatures, the oxides composition is uniform with depth, but there is an increasing depletion of In near the substrate interface as the growth temperature increases.