Species formed during the decomposition of silane-methane-hydrogen mixtures ([SiH4]x:[CH4]y:[H2]z), by spatial plasma separation technique using the TCDDC (Two Consecutive Decomposition and Deposition Chambre) systeml, are evaluated by mass spectrometric analysis and related with the structural and electro-optical properties for either amorphous (a-) or microcrystalline (μc-) thin films. Results obtained show that in the plasma region, the main reaction is the direct fragmentation of SiH4 by electron impact whilst near the growing surface, the main detected species are excited. The kind of species and their intensity depend strongly on the power density (dp), mixture gas ratio (g = CH4 /SiH4+CH4), static electromagnetic (ξ, B) fields and r.f. frequency (f), used. Since CH4 has a threshold decomposition higher than that one of SiH4, the species presented at the plasma region are, mainly, methyl, dimethyl and CH2-CH2 graphitic-like chains depending, mostly, on dp and on g. By diluting the mixture in H2, we observe the existence of active H2 species that, for high dp, may lead to a transition from the amorphous to microcrystalline phase, as well as a carbon incorporation in the amorphous tissue as graphitic-like bonds. This allow us to infer the merit of the TCDDC system in producing a-/μc-thin films that can be applied to photovoltaic and other applications.