Reflectance anisotropy (RA) spectra of InP(100) surfaces were taken either in the MOCVD environment or, after contamination-free transfer to ultra-high vacuum (UHV), in UHV down to 20 K. They were correlated with photoemission and Auger electron spectroscopy measurements to investigate the transition from the P-rich to the In-rich surface reconstruction. The strongest surface stoichiometry-induced changes were found in the RA-spectra at around 3 eV photon energy, i.e. in the range of the surface-modified bulk E-1 transition. At around 1.8 eV, the main peak of a pronounced surface transition was recorded with equal magnitude and sign for the P-rich and for the In-rich surface reconstruction. Two different specific RA-spectra measured with the highest peaks are postulated here to indicate the ordered P-rich and ordered In-rich surface reconstruction, respectively.