The plasma modification of polydimethylsiloxane (PDMS) thin films was studied by means of Fourier Transform Infrared Reflection Absorption Spectroscopy (FT‐IRRAS). The spin‐coated PDMS films (10–175 nm) were prepared on aluminium coated glass substrates, and their thickness was measured by spectroscopic ellipsometry. The direct hydrogen RF plasma interaction with PDMS is compared with the hydrogen plasma radiation, only. The evolution of the thin film IR absorption spectrum was monitored, and the changes of PDMS characteristic absorption bands as well as the formation of new bands are discussed. In particular, the appearance of a new band at 1 230 cm−1 was observed, and it was identified as LO phonon band of SiOx.