The interfacial characteristics and electrical properties of as-deposited and annealed Al2O3 gate dielectric films fabricated on fully depleted SiGe on insulator are investigated. The surface morphology of the gate dielectric is observed by atomic force microscopy, and its physical thickness and structure are determined by high-resolution transmission electron microscopy. Assessment of the energy shifts of the interfacial components observed by high-resolution x-ray photoelectron spectroscopy shows that oxidation of Ge occurs readily at the growth temperature, leading to a mixture of Si and Ge oxides at the Al2O3/SiGe interface. After annealing, the relative intensity of GeOx diminishes significantly, whereas the relative intensity of the Si suboxides or SiO2 increases, and especially, the formation of silicate is observed. The chemical state changes in the interfacial layer affect the flatband voltage (Vfb) and the density of the interfacial states.