We examine the homo- and heteroepitaxial growth of moderately thick (similar to700 nm) layers of GaSb with the objectives of optimizing growth conditions and determining the initial phase of heteroepitaxy on (001)GaAs. Real-time spectroscopic ellipsometry (RTSE) data show that the (001)GaSb surface degrades immediately in excess trimethylgallium (TMG), but both (001)GaSb and As-terminated (001)GaAs surfaces are stable in trimethylantimony (TMSb). The surface-dimer contribution to the optical-anisotropy (OA) signal of (001)GaSb is small and generally masked by structural (roughness) effects, hence it is not useful for determining surface stoichometry. However, we show that laser light scattering (LLS) data do allow the V/III ratio to be fine-tuned during growth to minimize macroscopic roughness. TEM micrographs show that our GaSb/GaAs heterointerface is relatively defect-free except for the necessary local accommodation of lattice mismatch. The initial phase of heteroepitaxy on (001)GaAs occurs here as a coexistence of separate regions of GaAs and GaSb. (C) 2004 American Vacuum Society.