GaAs nanowires (NWs) are grown on GaAs (1 1 1) B substrates in a molecular beam epitaxy system, by Au-assisted vapor-liquid-solid growth. We compare the characteristics of NWs elaborated with As-2 or As-4 molecules. In a wide range of growth temperatures, As-4 leads to growth rates twice faster than As-2. The shape of the NWs also depends on the arsenic species: with As-4, regular rods can be obtained, while pencil-like shape results from growth with As-2. From the analysis of the incoming fluxes, which contributes to the NWs formation, we conclude that the diffusion length of Ga adatoms along the NW sidewalls is smaller under As-2 flux as compared to that under As-4 flux. It follows that As-2 flux is favourable to the formation of radial heterostructures, whereas As-4 flux is preferable to maintain pure axial growth. (C) 2010 Elsevier B.V. All rights reserved.