Via a particular postgrowth annealing procedure applied to heteroepitaxial GaP films grown on Si(100) substrates by metal-organic vapor phase epitaxy, ex situ atomic force microscopy (AFM) provides insight into the spatial distribution of antiphase domains (APDs). On a specific sample, the AFM characterization reveals a variation of the APD concentration over the sample's surface. In situ reflectance anisotropy spectroscopy (RAS) was used as a complementary technique for the quantification of APDs on the P-rich prepared GaP surface. Besides the expected linear reduction of the RAS signal according to the presence of antiphase disorder, the comparison with the reflection anisotropy (RA) spectrum of an identically prepared, (2 X 2)/c(4 X 2) reconstructed surface of a homoepitaxial GaP(100) reference revealed further characteristic deviations. In principle, they originated from the additional reflection at the GaP/Si(100) heterointerface. Mainly, its interference with the surface reflection affected the normalization of the RAS signal. Corresponding interference corrections improved the agreement of the GaP/Si(100) RA spectra with the homoepitaxially grown GaP(100) reference in general and, in particular, the accuracy and reliability of the in situ APD quantification via RAS. Finally, the statistical evaluation of comprehensive ex situ AFM characterization agreed well with RAS results after interference corrections, which represent an in situ technique for the APD quantification over a macroscopic spot size of a few millimeters. (C) 2010 American Vacuum Society. [DOI:10.1116/1.3466529]