Au-catalyzed self-assembly of GaAs nanowires on (1¯1¯1¯)B GaAs by metalorganic vapor phase epitaxy is reported between 375 and 500°C, using tertiarybutylarsine and trimethylgallium in H2. The nanowires are [1¯1¯1¯]B aligned and kink-free. Below 425°C the nanowires have narrow base diameter distributions, closely matching the size (∼60nm) of the Au nanoparticles at their tip (no tapering). Above 425°C the nanowires show a hexagonal-based pyramidal shape with base edges normal to the ⟨2¯11⟩ in-plane substrate directions and base diameters which increase exponentially with temperature, indicating a kinetics limited growth along the nanowire sidewalls. Activation energies in the range of ∼20–23kcal∕mol were estimated for growth along both the sidewalls and the [1¯1¯1¯]B direction.