Vapour-phase epitaxy of GaAs in conditions where the growth is limited by the chemical reactions of the gases with the substrate and not by gas transport is correctly understood in case the gases are produced by the decomposition of a GaAs source by H2O. Using secondary ion mass spectroscopy we have measured the efficiency at which various impurities (C, O, Si, S, Zn, Mg, Cr and Fe) are transported from the source into the grown layer. It is found that impurity transport is driven by the formation of volatile oxides, allowing to foresee which impurity can or cannot be incorporated into the grown layer.