The effect of the group-V termination of GaAsSb(100) surfaces on the sharpness of InP/GaAsSb heterointerfaces was investigated. Lattice matched GaAsSb/InP(100)-layers were grown by metalorganic vapor phase epitaxy (MOVPE). Contamination free sample transfer from the MOVPE reactor into ultrahigh vacuum (UHV) allowed for the correlation of in-situ reflectance anisotropy/difference (RA/RD) spectra with low energy electron diffraction (LEED) and photoelectron spectra (XPS/UPS). The in-situ RA spectra indicated that the GaAsSb surface was Sb-rich during growth and turned preferably into an As-rich surface after growth. With LEED and XPS the Sb-rich and As-rich surfaces were correlated with (4×3) and c(4×4) symmetries, respectively. These are well known reconstructions from the related binaries GaAs and GaSb. The study of the InP/GaAsSb interfaces compared the two ordered GaAsSb(100) surfaces as templates for InP growth. XPS measurements of InP/GaAsSb interfaces taken in UHV and I-V curves of InP/GaAsSb resonant tunneling diodes indicated that Sb segregation into a subsequent InP layer was significantly lower when the InP film was grown on the c(4×4) reconstructed GaAsSb surface compared to the (4×3) reconstructed surface.