The chemistry and composition of grain boundaries in polycrystalline GaAs grown by liquid phase epitaxy (LPE) and molecular beam epitaxy (MBE) are examined using complementary Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS) and x‐ray photoelectron spectroscopy (XPS). The effects of an unintentional residual‐oxygen partial pressure during LPE growth are investigated in terms of grain boundary passivation. Depth‐compositional data verify the grain boundary localization of oxides using aninsitu, UHV fracturing technique. Indications of distributions of these oxides over the grain boundary are presented. The performances of Au Schottky barrier solar cells fabricated from the polycrystalline LPE and MBE GaAs are compared and differences are explained in terms of grain boundary activity.