The main scope of this article was to demonstrate the potential of Fourier transform infrared (FTIR) spectroscopy as a diagnostic tool for low-pressure plasma polymer modification. Two model polymers, polyethylene and polystyrene, were treated in radio-frequency(rf) discharges in argon, hydrogen, oxygen, nitrogen, and tetrafluoromethane. In situ FTIR analysis was based on infrared reflection absorption spectroscopy (IRRAS) and attenuated total reflection (ATR). For both techniques, thin-film samples were prepared from polymer solution. The effects appearing in the infrared spectra are discussed. The results are compared with those from other diagnostic techniques.