High resolution transmission electron microscopy, high angle annular dark field atomic imaging and energy-dispersive spectroscopy studies were performed on pseudomorphic ZnSe/GaAs (OOl) heterostructures grown using the two different interface fabrication procedures that produce epilayers with minimum native defect densities. ZnSe/GaAs heterostructures represent important prototype heterovalent materials systems to investigate the relation between the microscopic structure and composition of the interface and the macroscopic properties relevant to device applications. Despite many efforts in the last ten years, the connection between important structural properties of the final heterostructure and the initial stages of interface nucleation and growth remains controversial. Samples were fabricated in a multi-chamber molecular beam epitaxy (MBE) system that included interconnected chambers for II-VI MBE …