Transmission electron microscopy (TEM) studies are presented for Au-assisted vapor-liquid-solid (VLS) nanowire growth of gallium arsenide/gallium phosphide (GaAs/GaP) axial heterostructures. The supersaturation of the liquid Au-III-V alloy droplet during MBE growth was found to have a profound impact on both the crystal phase and growth rate of the nanowire heterostructures. Among these effects was the appearance of a previously unreported 4H GaP crystal phase. 4H GaP occurred at low Au-III-V droplet supersaturation following the transition from GaAs to GaP growth. Both crystal phase and growth rate were different for GaAs and GaP, underlining the importance of group V elements in Au-III-V droplet supersaturation and thus VLS nanowire growth. (C) 2011 Elsevier B.V. All rights reserved.