We report on the change of chemistry involved by molecular nitrogen in the deposition process of pp‐HMDSO thin films with an AP‐DBD, using HMDSO as chemical precursor. By modifying the composition of the main gas from pure argon to pure nitrogen, thin films composition varied from SiOC:H to SiOCN:H. A small amount of nitrogen favours polymer chain propagation, by consuming species responsible for chain termination and playing a role in propagation phase. Higher nitrogen content leads to more cross‐linked coatings. The use of optical emission spectroscopy together with FT‐IR and XPS is shown to be relevant to study such processes.