Plasma polymerized organosilicon thin films were deposited in a MW-PECVD using hexadimethylsiloxane (HMDSO) as a precursor. Thin films were synthetized with different plasma conditions ranging from soft (low plasma energy) to hard (high plasma energy) ones. Quantitative 29Si solid state NMR and FTIR spectroscopy were used to characterize the bulk composition. X-ray photoelectron spectroscopy (XPS) coupled with Density functional theory (DFT) calculations wereused to determine the surface chemicalcomposition.The bulk and the surface of the plasma polymerized HMDSO (PP-HMDSO) showed acomplex chemical composition. The four main environmentsM,D,T and Q were present in the films. An additional environment Si(CH2)(CH3)3 denoted S was detected in the PP-HMDSO films. The chemical composition comparison showed a difference between the bulk and the surface of the films.Under soft plasma conditions, a high amount of SiOC3termination waspresent in the bulk of the films. While, the SiO2C2chains were highly present on the surface. On the other hand, under hard plasma conditions, the numberof the SiOC3terminationincreasedon the surface and decreasedin the bulk. Under soft plasma conditions,thePP-HMDSOstructure was close to PDMS, whileunder hard plasma conditions, the PDMS similarity was lost.