Thin a-SiOxCyHz membranes were deposited from two different organosilicon precursors, a linear one the octamethyltrisiloxane (MDM) and a cyclic one the hexamethylcyclotrisiloxane (D3), in a low-frequency plasma polymerization process. The chemical structure of polysiloxane plasma materials was characterized using two different spectroscopic analyses: the X-ray Photoelectron Spectroscopy (XPS) and the Si-29 solid-state Nuclear Magnetic Resonance (NMR). Both techniques show that it is possible to synthesize a wide range of materials, more or less organic and close to silicone-type conventional polymers, depending on whether the energetic character of the plasma is low or high. The structural differences between plasma films synthesized from MDM (PP-MDM films) and plasma materials deposited from D3 (PP-D3 films) are all the more pronounced that the plasma conditions are soft due to the better preservation of the monomer chemical structure, The NMR analysis allows to display the presence of cyclic siloxane chains in PP-D3 layers whereas PP-MDM materials are exclusively composed of linear siloxane chains.