Surface diffusion of Sb on Si(111) has been studied by second harmonic microscopy, which uses surface second harmonic generation to monitor surface concentration profiles with a 3 mu m spatial resolution. At temperatures near 55% of the bulk melting point and in the coverage range 0 < theta < 0.12, the activation energy, E(diff), and pre-exponential factor, D-0, were found to be 60 +/- 3 kcal/mol and 6 x 10(3+/-0.7) cm(2)/s, respectively. The high prefactor and activation energy indicate that the surface diffusion is governed by a recently developed adatom-vacancy mechanism.