GaInNAs is potentially useful in a wide variety of devices, including next-generation, high efficiency, multijunction solar cells. Unfortunately, a number of defects exist in this material that limit the overall device performance. Beam-based positron annihilation spectroscopy was used to examine vacancies in GaInNAs grown by both metal-organic chemical vapor deposition (MOCVD) and molecular-beam epitaxy (MBE). A significant concentration of vacancies exists only when hydrogen and nitrogen are both present during growth. This finding supports the theoretical prediction that gallium vacancies occur in complexes with nitrogen and hydrogen. Growing GaInNAs by solid-source MBE, a hydrogen-free growth method, minimizes the formation of the vacancy complexes. Because this complex is an acceptor, it may be a cause of the high background acceptor concentrations in MOCVD-grown GaInNAs. (C) 2004 American Vacuum Society.