A 2.45 GHz electron cyclotron resonance reactor was used to deposit silicon oxide films using both tetraethoxysilane (TEOS)/O-2 and silane/O-2. The reactor is equipped with an in situ microwave spectrometer in the frequency range 75-110 GHz,that is used to monitor the gas phase chemical species in this environment. The J = 1 --> 2 rotational transition of the vibrational ground state of silicon monoxide (SiO) was detected and monitored in plasmas of both gas mixtures. The integrated intensity of the microwave transition was used to obtain absolute densities of SiO integrated along the path length for both of these plasma chemistries. Silicon oxide films were deposited at pressures of a few mTorr and at low wafer temperature. The properties of the deposited films were characterized using infrared absorption and wet etch rate. The refractive indices of the films were measured by using multicolor ellipsometry. Deposition rates were determined as a function of oxygen/TEOS or oxygen/silane flow ratio. An inverse correlation was found for both sources between SiO abundance and deposition rate. An inverse correlation between SiO abundance and film quality was observed for the silane source.