GaAs nanowires were synthesized by molecular beam epitaxy on different substrates using Mn as catalyst. High density of 1D nanowires was obtained at growth temperature between 540 and 620 C on SiO 2 substrates. On oxidised GaAs substrates nanowires grow together with 2D nanostructures that are dominant. Nearly no nanowires were instead obtained on epitaxial GaAs. The nanowire yield on the different substrates is correlated to the chemical reactions taking place between substrate and catalyst before the growth, as detected by x-ray photoelectron spectroscopy.