GaAs nanowire (NW) heterostructures with four GaAsSb inserts were grown on GaAs(111)B substrates by Au-assisted molecular beam epitaxy (MBE). Structural characterization of the NWs showed that the transition between the GaAs and GaAsSb heterojunction is atomically abrupt and that the GaAs crystallographic phase is mainly wurtzite with some stacking faults, whereas the GaAsSb phase is zinc blende and free from twinning defects. Interestingly, the growth rates of the GaAs segments and GaAsSb inserts were observed to vary with growth time. We attribute this behavior to the effect of the inclined molecular beams with respect to the substrate surface in the MBE growth chamber. Further, the mole fraction of Sb in the GaAsSb insert was determined by energy-dispersive X-ray spectrometry and was found to increase with the inserts position along the NW. (C) 2008 Elsevier B V All rights reserved.