Our recent studies were concerned with the microwave plasma reactions of imidazole molecules to cross-linked poly(dimethylsiloxane) (PDMS) surfaces. However, the issue of the PDMS surface sites responsible for imidazole bonding reactions remained open. In this study, attenuated total reflectance (ATR) Fourier transform infrared (FT-IR) spectroscopy will be used to assess the origin of species responsible for the PDMS reactions with imidazole. Analyses of spectroscopic data indicate that the primary reactive sites on the PDMS surfaces are Si-CH2. radicals, which are formed through the hydrogen abstraction of the Si-CH3 groups. The Si-CH2. radicals react with the imidazole CHCHNCHN. radicals, resulting from hydrogen abstraction of the N-H bonds, to form Si-CH2-imidazole surface entities. Reaction mechanisms responsible for imidazole-PDMS reactions in microwave plasma are proposed.