Reflection anisotropy (RA) spectroscopy has been used to examine the in vacuo (001) surface of InP before and during growth by molecular beam epitaxy (MBE). The dominant effect on the RA signal occurring the initiation of growth is the change in the surface V/III ratio, caused by the exposure of the surface to the incident indium flux. During MBE growth of InP under commonly used conditions, RA oscillations are clearly observed. These oscillations have been confirmed to correspond to the growth of InP monolayers. The oscillations are tentatively ascribed to the variation in P coverage during the growth of each monolayer of material. © 1998 American Institute of Physics.