A high resolution core level photoemission study on the metal contact formed on S-passivated n-InGaP(100) surface reveals that the species produced by the chemical reaction of the deposited metal atoms with the surface atoms induce the gap states responsible for the Fermi level movement. The initial sulfur passivation of n-InGaP(100) surface efficiently reduced the gap states within the band gap and flattened the band bending by 0.6 eV relative to the sputter-cleaned surface. When the metals such as Al(Au) are deposited on the S/InGaP(100) surface, it was found that the deposited Al(Au) reacted with monosulfides resulting in the Al-sulfide (the remaining defective In-polysulfides) which seem to be the origin of the gap states.