For an indirect in situ quantification of Si(1 0 0) surface reconstruction domains, we characterized the anti-phase disorder of thin, pseudomorphic gallium phosphide films after initial heteroepitaxy on different Si(1 0 0) substrates by reflectance anisotropy spectroscopy and conducted transmission electron microscopy and atomic force microscopy for comparison. The well-defined preparation of the well-established P-rich reconstruction on the surfaces of our GaP/Si(1 0 0) samples enabled an advanced quantitative analysis of the optical in situ spectra. In reference to a homoepitaxial GaP(1 0 0) sample with identical surface preparation, we first corrected the influence of interfacial reflections according to the relative reflectance and, then, determined the anti-phase domain content of the films by a signal intensity analysis. Due to specific growth conditions chosen for straight propagation of the GaP anti-phase boundaries after initiation at single-layer or odd numbered steps of the Si(1 0 0) surface, the in situ results confirmed the domain distributions on the SO 0 0) substrates that have been observed by ex situ atomic force microscopy. Subsequent structural analysis of the anti-phase boundaries by specific dark-field transmission electron microscopy techniques verified their straight growth in cross section images, while plan-view observations reproduced the anticipated Si(1 0 0) surface structure in very much detail. (C) 2010 Elsevier B.V. All rights reserved.