ZnO nano-particles have been synthesized in low pressure flow reactors utilizing Zn(CH3)(2) as precursor. Two different synthesis routes have been employed. A low pressure flame reactor and a microwave reactor were used for synthesis of ZnO particle in Zn(CH3)(2) doped H-2/O-2/Ar flames and Zn(CH3)(2) doped Ar/O-2 plasmas, respectively. The particle formation process has been investigated in situ by a particle mass spectrometer. Also, sampled powders have been investigated ex situ by means of FT-IR, XRD, TEM, and UV-VIS. For both synthesis routes nanometer sized ZnO particles were found with particle diameters in the range between 4 to 8 nm. In cases of the flame reactor the results suggest a strong influence of water on the particle formation process.