Dissociations in plasma enhanced chemical vapor deposition for SiO2 deposition using tetraethoxysilane (TEOS) were investigated by means of mass spectrometry and optical emission spectroscopy. First, we showed mass spectra and optical emission spectra in discharge of TEOS and He mixture. It was shown that CO molecules are produced by electron impact dissociation of TEOS without oxidant gas. Next, we performed isotopic labeling studies using O-18(2) in order to clarify a role of added oxygen for dissociation of TEOS. The mass spectrometric signal with isotope shifts showed that (CO)-O-18, (COO)-O-16-O-18, (CO2)-O-18, (OO)-O-16-O-18 and (H2O)-O-18 were main products of oxidation promoting dissociation of TEOS. It is found that oxygen gas plays an indispensable role for dissociation of TEOS in the plasma.