Ge_{1-x}Sn_{x} nanoparticles are interesting for a variety of different optoelectronic devices, however, the synthesis normally involves highly inert conditions, making it less available and promising for future industry implementation. Here, a new non-inert synthesis route is presented which involves preparation of the synthesis under ambient conditions followed by a reaction in autoclaves at temperatures between 400 °C and 500 °C and pressures between 52 bar and 290 bar. The product formation is also investigated with in situ powder X-ray diffraction (PXRD) to study the effect of the reaction parameters in more detail, e.g. showing that the Sn-precursor catalyzes the reaction. The synthesized phase pure Ge_{1-x}Sn_{x} nanoparticles have Sn concentrations ranging from 0 to ∼4% and crystallite sizes ranging from approximately 11 nm to 25 nm. If the Sn-precursor concentration is increased further, β-Sn is formed as an impurity phase accompanied by an increase in the size of the Ge_{1-x}Sn_{x} particles, making sizes of up to about 55 nm available.