We measure the activation energy for surface diffusion of isolated Pb adatoms on Ge(111) by observing individual atomic interchanges using a scanning tunneling microscope. The diffusion occurs mainly along one direction of the c(2×8) reconstructed surface with Pb adatoms moving between substitutional Ge adatom sites. Only half of the adatom interchanges are to near-neighbor sites, the rest are long jumps. The scanning process does not affect the diffusion. For temperatures from 24 to 79°C we find the diffusion obeys an Arrhenius law with an activation energy of 0.540.03 eV.