Si and Si1-xGexnanowires are of interest for nanoscale electronics, sensors and photovoltaics. These structures are commonly formed by chemical vapor deposition via a metal-mediated vapor-liquid-solid mechanism using SiH4and GeH4sources. The effect of growth conditions on the growth rate and composition of Si1-xGe xwas investigated. This work reveals a difference in the mechanism of incorporation of Si into Si1-xGexnanowires compared to Si nanowire growth which is believed to arise as a result of gas phase interactions between SiH4and GeH4. The growth of Si 1-xGex/Si axial heterostructured nanowires was also investigated. Compositional analysis indicates that the Ge profile exhibits turn-on and turn-off transients which are associated with the time required to saturate and deplete Ge from the Au-Si-Ge catalyst. This leads to an increase in the interfacial width with increasing nanowire diameter.