Selective area epitaxy of InP on masked (100) InP substrates is studied. InP layers are deposited between pairs of SiO2 stripes using low-pressure metalorganic chemical vapor deposition. Layer thickness is investigated by surface profiling and scanning electron microscopy. For growth between oxide stripes, the growth velocity is enhanced by lateral diffusion of growth species from the masked region to the exposed region. Two transport mechanisms are known to exist: vapor phase diffusion and surface migration. However, most existing quantitative models focus only on the former. A new computational model, based on the diffusion equation with time dependent boundary conditions, is presented which describes the growth enhancement component due to surface migration. The role played by surface migration is shown to depend on nominal film thickness. The model correctly predicts a super growth enhanced region adjacent to the oxide. Previous quantitative models have not successfully described this aspect of growth near the oxide film.