Surface diffusion of In and Sb on Ge(111) has been measured by optical second harmonic microscopy. This technique employs surface second harmonic generation to directly image submonolayer surface concentration profiles. At low coverage, the activation energy Ediff and pre-exponential factor D0 are extraordinarily large for both systems and cannot be explained by theories based on simple site-to-site hopping. Molecular dynamics simulations of a Lennard-Jones(111) surface has revealed a new mechanism for high-temperature diffusion which is probably operative on Ge(111). At low coverages and high temperatures, adatom-vacancy pairs. This process is characterized by a high enthalpy of pair formation and a high entropy of activation due to lattice relaxation near the vacancy. At high coverages, the behaviour of Sb and In are quite different. As the In coverage increases, a larger fraction of In resides in the adlayer, so the diffusion parameters slowly approach those for site-to-site hopping. For Sb there is evidence for immobile islands at higher coverages. This does not effect Ediff but decreases D0 due to the smaller number of diffusion paths around the island. The gradual change of In diffusion parameters with coverage is interrupted by an apparent phase transition at θ=0.16, where both Ediff and D0 peak sharply.