Fluorescence-mode XAFS has been used to study the local environment about chosen atomic species such as Ga and As in bulk oxide Al(1-x)Ga(x)As (x=0.96) and at the interface between thin (300 A) oxidized Al(1-x)Ga(x)As (x=0.94) film and GaAs substrate in total external-reflection mode. X-ray reflectivity experiments have also been employed to investigate the density profile of the oxide film on a GaAs substrate revealing the density profile as a function of depth. It is important to find out how the As is incorporated at the interface, the interfacial strain, and related local structural parameters for understanding that may be central in developing high performance III-V MOSFET devices.