The crosslinking of thin liquid PDMS layers by three different technically relevant processes, H2 radio‐frequency plasma treatment, Xe2* excimer VUV irradiation, and low‐energy electron beam processing is investigated. The modifications to the layers due to the processing are monitored by means of RAIRS. Plasma processing of liquid PDMS leads to a direct conversion to a SiOx‐like material of the topmost layers, whereas a gradual transition from PDMS to the same product is observed upon VUV irradiation. Electron exposure does not induce oxidation. The initiating steps of the conversion induced by the interaction with VUV photons, low‐energy electrons, or their combined effect with ions and H atoms in the plasma are discussed. The latter creates a high density of damage sites. magnified image