Thin InP layers were grown by metalorganic vapor phase epitaxy on the ternary compound GaAs0.5Sb0.5 lattice matched to InP(1 0 0). The heterojunctions were studied with in situ reflectance anisotropy spectroscopy and benchmarked in ultrahigh vacuum with ultraviolet and X-ray photoelectron spectroscopy and low energy electron diffraction with regard to the sharpness of the interface. During growth of GaAs0.5Sb0.5 an Sb-rich,(1 x 3)-like reconstruction was observed and during stabilization with TBAs an As-rich c(4 x 4) reconstruction. These two different reconstructions of GaAs0.5Sb0.5(1 0 0), well-known from the binaries GaSb(1 0 0) and GaAs(1 0 0) respectively, were used for preparing InP/GaAS(0.5)Sb(0.5) heterojunctions. The RA spectra of thin heteroepitaxial InP layers were compared to a well-established RA spectrum of MOVPE-prepared homoepitaxial, (2 x 1)-like reconstructed P-rich InP(1 0 0), that was used as a reference spectrum of a well defined surface. Growing InP on the c(4 x 4) reconstructed GaAsSb(1 0 0) surface resulted in a significantly sharper interface than InP growth on (1 x 3) reconstructed GaAsSb(1 0 0)..1 (c) 2005 Elsevier B.V. All rights reserved.