Polystyrene-like coatings are synthesized by plasma near atmospheric pressure. Elucidating their chemical structure after exposure to ambient air could be very challenging because of the interference of surface-related phenomena, mainly post-polymerization oxidation and contamination. In this paper, we propose secondary ion mass spectrometry (SIMS) in molecular depth-profiling mode, combined to multivariate analysis, as a more reliable tool for their investigation as a function of the injected power. Indeed, the information provided by the inner layers is more representative of the film in growth. The SIMS approach is validated by complementary, surface-sensitive and bulk techniques: X-rays photoelectron spectroscopy (XPS) and infrared spectroscopy (IR). The SIMS results suggest that the high concentration of -CH3 groups in the polymer matrix, pointed out by IR, is due to branching and/or grafting of CH3 center dot radicals to active sites (prevalently in position alpha, beta, gamma) along the aliphatic backbone, in addition to a significant fraction of trapped oligomers. The oligomer contribution is supported by an original study based on the molecular weight dependence of the sputtering efficiency. The overall experimental evidences indicate a milder fragmentation of the precursor at lower powers, leading to a higher conservation of the aromaticity and a lesser branched and/or cross-linked content.