We report on the gas phase synthesis of highly crystalline and homogeneously alloyed Si_1−xGe_x nanocrystals in continuous and pulsed plasmas. Agglomerated nanocrystals have been produced with remarkable control over their composition by altering the precursor GeH₄ gas flow in a continuous plasma. We specially highlight that in the pulsed plasma mode, we obtain quantum-sized free standing alloy nanocrystals with a mean size of 7.3 nm. The presence of Si_1−xGe_x alloy particles is confirmed with multiple techniques, i.e. Raman spectroscopy, XRD (Xray diffraction) and HRTEM (high resolution transmission electron microscopy) studies, with each of these methods consistently yielding the same composition. The nanocrystals synthesized here have potential applications in band-gap engineering for multijunction solar cells.