The adsorption position and ordering degree of S atoms on a S-passivated InP(001) surface were investigated by using soft-x-ray standing waves (XSWs) in the photoelectron collection mode. Soft-x-ray photoelectron spectroscopy for a (NH4)(2)S-x-treated and de-ionized water-rinsed InP(001) surface revealed that a small amount of residual elemental S atoms on the InP(001) surface was completely removed by postannealing at 400 degrees C in a vacuum. XSW results for the annealed sample showed that most of the S atoms stayed at the bridge site on the In-terminated InP(001) surface, forming In-S-In bridge bonds.