Employing either nitrogen or hydrogen as carrier gas, the preparation of GaP and InP (1 0 0) surfaces by metal-organic vapor phase epitaxy (MOVPE) was monitored in situ by reflectance anisotropy/difference spectroscopy (RAS/RDS). The development of the characteristic surface reconstructions differed significantly dependent on the MOVPE ambient, N(2) VS. H(2). In addition to specifically ordered, well-established P-/H-terminated and cation-terminated surface reconstructions, intermediate surface phases were identified by characteristic RA spectra, when preparing GaP(1 0 0) in N(2). In contrast, InP(1 0 0) showed very similar surface signals independent of the utilized MOVPE process gas. The experimental results were compared with the established surface phase diagrams obtained from first-principles total-energy and electronic structure calculations. For verification of the surface symmetry attributed to the different characteristic in situ RA spectra, the findings were benchmarked with low energy electron diffraction measurements after contamination-free transfer of the samples to ultra-high vacuum. (C) 2010 Elsevier B.V. All rights reserved.