InAs nanowires were grown on GaAs substrates by the Au-assisted vapour-liquid-solid (VLS) method in a gas source molecular beam epitaxy (GSMBE) system. Passivation of the InAs nanowires using InP shells proved difficult due to the tendency for the formation of axial rather than core-shell structures. To circumvent this issue, AlxIn1-x or AlxIn1-xP shells with nominal Al composition fraction of x=0.20, 0.36, or 0.53 were grown by direct vapour-solid deposition on the sidewalls of the InAs nanowires. Characterisation by transmission electron microscopy revealed that the addition of Al in the shell resulted in a remarkable transition from the VLS to the vapour-solid growth mode with uniform shell thickness along the nanowire length. Possible mechanisms for this transition include reduced adatom diffusion, a phase change of the Au seed particle, and surfactant effects. The InAs-AllnP core-shell nanowires exhibited misfit dislocations, while the InAs-AllnAs nanowires with lower strain appeared to be free of dislocations. (c) 2012 Elsevier B.V. All rights reserved.