In situ diagnostic investigations have been performed by using mass and infrared spectroscopy during low-temperature plasma-enhanced chemical vapor deposition of silicon-oxide films using tetramethylsilane and oxygen as reactants. Mass analysis reveals that almost reactant organosilicon molecules are dissociated in the plasma Si+ and SiO+ are the fragments mainly detected juring deposition. The direct precursors in the silicon-oxide depostition are deduced to be the radicals which can be dissociated into Si+ or SiO+ in the dissociative ionization process in the mass spectrometer. Mass spectroscopic monitoring of oxygen has been proposed as an easy and useful method to find an appropriate condition for pure silicon-dioxide deposition. Existence of Si-H bonds on the film surface and/or in vapor phase during deposition has been found by infrared diagnostics.