Si1-XGex nanowires (NWs) constitute promising building blocks for future electronic and optoelectronic devices due to the enhanced tuneability of their physical properties, achieved mainly by controlling their chemical composition. In this study, the pressure dependence of the chemical composition, growth and tapering rates and crystalline structure of Si1-XGex NWs grown by the CVD-VLS technique was investigated. It is demonstrated for the first time, that the composition of single crystal Si1-XGex NWs can be readily modulated between ca. x = 0.75 to x = 0.25, simply by altering the total growth pressure while keeping all other growth parameters fixed. Moreover, this procedure does not cause any undesired structural or morphological side effects. Growth pressure is hence concluded to be the most significant parameter for tailoring Si1-XGex NWs electron and phonon mobility, band gap, and so forth. The observed alloy-composition control phenomena can be explained by the interplay between the pressure-dependent unimolecular decomposition of the individual precursor gases, SiH4 and GeH4, at the given experimental conditions that leads to a direct modulation of the decomposed/activated Si/Ge precursors ratio in the gas feedstock and is finally reflected in the composition of the obtained binary alloy nanowires. In addition, a silicon-germanium cooperative growth mechanism is suggested to account for the observed growth rate pressure dependence and enhanced growth rates.