Abstract Cu has been reported to diffuse rapidly in GaAs at low temperatures (2.3×10-5 cm2 s-1 at 600°C). The rapid diffusion is attributed to the interstitial movement of Cu atoms. The present investigation was undertaken to examine preferential diffusion of Cu+ along dislocations and grain boundaries in GaAs. The experiments consisted of depositing 64Cu on a GaAs water surface, annealing in vacuum, and observing the Cu distribution by autoradiography. From these observations no preferential diffusion along dislocations or grain boundaries was detected in SI GaAs annealed between 600 and 1000°C. In the sample annealed above 800°C, the deposited Cu reacted with the GaAs forming a liquid on the sample surface, which solidified into complex Cu-Ga-As compounds. The liquid also produced Cu-rich pipes which extended through the GaAs wafer.