We have investigated adsorption and dissociation of water and HfCl4 on Ge/Si(100)-(2×1) surface with density functional theory. The Si–Ge heterodimer and Ge–Ge homodimer are employed to represent the Si1-xGex surface. The activation energy for adsorption of water on Ge–Ge homodimer is much higher than that on Si–Ge heterodimer. No net activation barrier exists during the adsorption of HfCl4 on both SiGe surface dimers. The differences in the potential energy surface between reactions on Si–Ge and Ge–Ge dimers are due to different bond strengths. It should also be noticed that the activation energy for HfCl4 is quite flat, thus HfCl4 adsorbs and dissociates on Ge/Si(100)-(2×1) easily.