We have studied the formation of the Sb/InP(100) interface by photoemi
ssion extended x-ray absorption fine structure. Our results indicate t
hat a low Sb coverage stretches and subsequently weakens P-In surface
bonds which appears as a precursor stage prior to In-Sb bond formation
. Then, at increasing Sb coverages, the substrate surface reconstructs
with bonds approaching the relaxed clean InP(100) surface values. Thi
s results from the breakdown of In clusters (formed during surface pre
paration by ion sputtering) which leads to the formation of an InSb in
terface layer. The surface is finally found to be stabilized at higher
Sb coverages with no more change in the substrate first and second ne
ar neighbor bond distances.