The authors report on the thermal stability of Ge metal-oxide-semiconductor (MOS) devices. Ge MOS capacitors with ZrO2 high-k gate dielectric and TaN metal gates were fabricated on Ge epitaxial films. Ge MOS capacitors exhibited a very low gate leakage current density of ∼1×10-6 A/cm2 with a capacitance equivalent thickness of 13 Å. The excellent electrical characteristics, however, degraded when Ge/ZrO2 gate stacks were subsequently annealed at elevated temperatures that are potentially used for transistor fabrication. The thermal degradation was due primarily to the formation of interfacial Ge oxides. Ge oxidation temperature was identified using surface analysis and correlated with electrical characteristics.