Abstract 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 × 3 ) -like reconstruction was observed and during stabilization with TBAs an As-rich c ( 4 × 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/GaAs0.5Sb0.5 heterojunctions. The RA spectra of thin heteroepitaxial InP layers were compared to a well-established RA spectrum of MOVPE-prepared homoepitaxial, ( 2 × 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 × 4 ) reconstructed GaAsSb(1 0 0) surface resulted in a significantly sharper interface than InP growth on ( 1 × 3 ) reconstructed GaAsSb(1 0 0).