We investigate the effects of various surfactants on the Ge adsorption, diffusion, and exchange on Si(111) surfaces through first-principles pseudopotential total-energy calculations. For surfactant-covered surfaces such as Si(111):Ga-1 x 1, Si(111):As-1 x 1,and Si(111):Sb-(root 3 x root 3)R30 degrees, Ge adatoms are generally incorporated into the surfactant layer. On the Ga-covered surface, we find-strong interactions between the Ge and Ga atoms, which result in large activation energies for both Ge surface diffusion and exchange with an underlying Ga atom. In the case of As surfactants, the activation energies for adatom diffusion and exchange are much reduced because of the weak couplings between the Ge and As atoms. Similarly, on the Sb-covered surface, the exchange between the adatoms and surfactants takes place easily; however, the surface diffusion is severely suppressed due to a relatively large energy gain by the exchange process. [S0163-1829(99)00128-9].