Nanowires (NWs) are promising for the integration of III-V compound-based electrical and optical devices on Si. Selective area growth (SAG) of InxGa1-xAs NWs with compositions x varying from 0.34 to 0.90 is achieved. NWs are grown on patterned Si(111) substrates by hydride vapor phase epitaxy (HVPE) at temperatures around 690 °C. The composition is measured using EDX profiles along their lengths and found to be quite homogeneous. Theoretical analysis revealed that the NW composition is kinetically controlled and well-fit by the one-parametric Langmuir-McLean formula, with no thermodynamic factors influencing the compositional trend. This study highlights the capability of catalyst-free HVPE SAG to grow highly uniform InGaAs NW arrays with a widely tunable composition on Si substrates.