Silicon oxide films were deposited at room temperature on Si substrates by remote-type plasma enhanced chemical vapor deposition using tetramethoxysilane and oxygen. Fourier transform infrared spectroscopy revealed that the carbon contamination in the deposited films are in the state of Si-CH3. Quantitative analysis by X-ray photoelectron spectroscopy elucidated that the oxygen partial pressure ratio is needed to be < 40% in order to synthesize pure silicon oxide films without the carbon contamination. Mass analysis showed that almost reactant organosilicon molecules were fragmented into small gaseous species in the plasma even without oxygen. The origin of the carbon contamination was adsorption of CH3 radicals onto the film surface. The existence of oxygen molecules in vapor phase resulted in the deposition of carbon-free silicon oxide films.