Arsenic precipitates have been observed in GaAs low-temperature buffer layers (LTBLs) used as "substrates" for normal molecular beam epitaxy growth. Transmission electron microscopy has shown the arsenic precipitates to be hexagonal phase single crystals. The precipitates are about 6±4 nm in diameter with a density on the order of 1017 precipitates per cm 3. The semi-insulating properties of the LTBL can be explained in terms of these arsenic precipitates acting as "buried" Schottky barriers with overlapping spherical depletion regions. The implications of these results on LTBL resistivity stability with respect to doping and anneal temperature will be discussed as will the possible role of arsenic precipitates in semi-insulating liquid-encapsulated Czochralski-grown bulk GaAs.