The surfaces of polyethylene (PE) films were modified by deposition of layers from acetylene/ethylene monomer gases in a low-pressure radio-frequency plasma. The chemical structure of the plasma-deposited layers and their long-term stability were studied by X-ray photoelectron spectroscopy (XPS), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and contact angle measurements. These studies have shown that the plasma-deposited layers consist mainly of amorphous, short-chain, functional CxHy structures with aryl units. As the reactive radical centers formed during the plasma process are susceptible to further reaction with atmospheric oxygen and water, the chemical stability of these layers was investigated. This stability is ensured over the long-term, although an increase in the number of functional groups is obtained over time. It was demonstrated that the post-reactions reach a state of equilibrium after a few weeks.