We fabricated Ge and Si metal oxide semiconductor devices with Pt/HfO2 gate stacks and investigated their structural and electrical properties. Postmetallization annealing in O-2 ambient reduced the accumulation capacitance more significantly in Si devices than in Ge devices due to the increase in the thickness of a low-k interfacial layer in between the HfO2 film and Si substrate. Ge devices exhibited lower effective work function values for a Pt gate electrode than Si devices owing to the presence of a large number of positively charged dipoles caused by a strong Fermi-level pinning at the Ge surface.