We have used Bi as a surfactant in Ge growth on Si(111) and present a detailed analysis of the adsorption site geometry of Bi on bare Si(111) as well as on ultrathin Ge films (1-2 monolayers, ML) grown on Bi-terminated Si(111). X-ray standing waves (XSWs) have been employed to show that at a growth temperature of 485 degrees C, Bi occupies T-1 adsorption sites on Si(111). After Ce deposition, a site exchange of Ge and Bi is observed, and Bi is found to reside on T-1 sites on top of the pseudomorphically strained Ge. The Si-Bi and the Ge-Bi bond lengths have been determined to be very close to the sum of the respective covalent radii.