The mechanism of the surface segregation and interdiffusion of Ge on Si(001) during epitaxial growth and annealing was studied using medium-energy ion scattering (MEIS). One monolayer of Ge was grown on Si(001) at 450 degrees C and annealed at 780 degrees C. The Ge was found to redistribute during the very early stages (within 1 min) of the annealing process and remained constant even after annealing at longer periods of time. Therefore, we conclude that the top few layers attain thermal equilibrium in a short period of time, such as less than 1 min. The coverage dependence of Ge distribution in the top few layers was also observed clearly from the surface of Ge deposited on Si(001) at 780 degrees C. This study strongly suggests that the subsurface phenomena should be considered in order to more accurately describe the Si/Ge interface structure.