Mass Spectrometry (MS) analyses, up to 300 amu, were performed on linear hexamethyldisiloxane (HMDSO) and cyclic octamethylcyclotetrasiloxane (OMCTSO) vapors in low frequency plasmas (40 kHz). The fragmentation rates of the precursors, and possible oligomerization in the gas phase, depend on the plasma power. Two extreme plasma power conditions were compared: a high plasma power density and a low plasma power density, corresponding to silica-like and polymer-like film deposition conditions, respectively. The results of ex-situ Fourier transform infrared spectroscopy (FTIR) analyses can be explained by the plasma composition analyzed by MS.