We report on the self-catalysed growth of vertical InAs1-xPx nanowires on Si(111) substrates by solid-source molecular-beam epitaxy. High-resolution transmission electron microscopy revealed the mixed wurtzite and zincblende structure of the nanowires. Energy dispersive x-ray spectroscopy and x-ray diffraction measurements were used to study the phosphorus content x in the InAs1-xPx nanowires, which was shown to be in the range 0-10 %. The dependence of phosphorus incorporation in the nanowires on the phosphorus flux in the growth chamber was investigated. The incorporation rate coefficients of As and P in InAs1xPx nanowires were found to be in the ratio 10 ± 5 to 1. © 2013 IOP Publishing Ltd.