The growth of InP nanowires by the VLS mechanism on Si <1 1 1> substrates with varying pre-growth treatments was investigated. When Au-catalyst particles were treated with trimethylindium before growth there was an increase in the fraction of catalyst particles yielding wire growth and in the number of these wires growing vertically from the substrate. This was confirmed using scanning electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. Cross-sectional transmission electron microscopy revealed that Au-catalyst particles exposed to PH3 before growth did not alloy with the underlying Si substrate. InP wires were also grown using Ag catalyst particles which do not alloy with the Si substrate. However, wire growth does not appear to be inhibited in this case. These results suggest that P is the primary cause for the both lack of growth from some catalyst particles and growth in directions other than the vertical <1 1 1> direction of the substrate. (C) 2008 Elsevier B.V. All rights reserved.