Because of the difficulties of using electron-based techniques in the metalorganic vapor-phase epitaxy (MOVPE) environment, an in situ X-ray diffractometer that combines a goniometer and reactor chamber was developed. Consequent measurements of P-rich InP(001) surface grown by MOVPE show the surface has a (2 x 1) structure. Calculations based on a P-dimer model suggest that this structure is composed of P-dimers whose bonding is parallel to the [(1) over bar 10] direction and indicates indium displacement in the second layer.