The interfacial reaction between thermally oxidized In1−xGaxAsyP1−yand an In1−xGaxAsyP1−yepilayer was studied using Raman and x‐ray photoelectron spectroscopy (XPS) analyses. In Raman spectra, it was found that the appearance of the phonon modes, i.e., the first‐order longitudinal (LO) and transverse‐optical (TO) modes for crystalline arsenic, was due to the In1−xGaxAsyP1−y‐oxide interfacial reaction. The XPS analyses showed that this reaction corresponded to the GaAs‐oxide interfacial reaction, i.e., As2O3+2GaAs→Ga2O3+4As. Furthermore, the reaction depends on the compositionyof In1−xGaxAsyP1−y, which may be due to the enhancement in the initial transient reaction by thermal damage of In1−xGaxAsyP1−yoccurring at the interface.