The interfacial reaction of hafnium-silicate [(HfO(2))(x)(SiO(2))(1-x), x=0.5,0.7] thin films grown on Ge(001) by atomic layer deposition was investigated using x-ray photoelectron spectroscopy and medium energy ion scattering spectroscopy. According to the peak changes in Hf 4f and Ge 3d, the Hf-silicate film reacted with the oxidized Ge surface forming Hf-germanate at the interface. The formation of Hf-germanate induced band bending of the Ge substrate at the interface and decreased band gap to 5.1 eV, which was lower than that of GeO(2) (5.6 eV). In particular, the interfacial reaction was dependent on the amount of SiO(2) in the Hf-silicate film, which resulted in more decrement in the band gap in the film with a high SiO(2) fraction.