The solid-state reaction between platinum and silicon nanowires grown by the vapor-liquid-solid technique was studied. The reaction product PtSi is an attractive candidate for contacts to p-type silicon nanowires due to the low barrier height of PtSi contacts to p-type Si in the planar geometry, and the formation of PtSi was the motivation for our study. Silicidation was carried out by annealing Pt on Si nanowires from 250 to 700 degrees C, and the reaction products were characterized by transmission electron microscopy. Strikingly different morphologies of the reacted nanowires were observed depending on the annealing temperature, platinum film thickness, silicon nanowire diameter, and level of unintentional oxygen contamination in the annealing furnace. Conversion to PtSi was successfully realized by annealing above 400 degrees C in purified N2 gas. A uniform morphology was achieved for nanowires with an appropriate combination of Si nanowire diameter and Pt film thickness to form PtSi without excess Pt or Si. Similar to the planar silicidation process, oxygen affects the nanowire silicidation process greatly.