The transition from tin monoxide to tin dioxide is investigated by doping a low pressure premixed H2/O2/Ar flame with dil. concns. of tetra-Me tin (TMT) Sn(CH3)4. The H2/O2 ratio was varied between 1.97 and 0.97; the reactor pressure was set to 30 mbar. The inlet gas velocity was kept const. and the precursor concn. was varied between 200 and 700 ppm. Mean particle diams. between 3 nm? dp? 7 nm were measured using a particle mass spectrometer (PMS), while X-ray diffraction (XRD) and transmission electron microscope (TEM) anal. revealed information on crystal structure as well as morphol. of the synthesized oxides. A gray SnO film was obsd. in the reactor at low oxygen concns., while for higher oxygen concns. light yellow sub-stoichiometric tin oxides and white SnO2 were obtained. Further anal. of the sub-stoichiometric oxides, using at. electron spectroscopy AES revealed that SnO2-x (0.2? x? 0.6) was obtained. Furthermore, elec. characterization of all materials, deposited on interdigital capacitors, showed a decrease in the cond. of the nanoparticles with increasing oxidn. (decreasing x).