Thermal-induced interfacial reactions and recrystallization of extrinsic phases in ZnO/GaAs heterostructures were observed by using high-resolution X-ray diffraction and transmission electron microscopy. A uniform Zn(3)AS(2) interlayer and a defected ZnO interlayer (i.e. embedded with nanoscale ZnGa2O4 domains) are formed, adjacent to each other along the growth direction, at elevated temperatures. The anomalous distribution of Zn and Ga elements over the two interlayers as confirmed by energy-dispersive spectroscopy conflicts with atoms interdiffusion driven only by concentration gradient. Chemical reactions between the diffused cations and the steady anions may accelerate the Ga/Zn atoms interdiffusion and result in the observed structures. (C) 2008 Elsevier B.V. All rights reserved.