Timeof-flight secondary ion mass spectrometry (ToF-SIMS) is used to monitor in a semi-quantitative way the chemical modifications produced at low-density polyethylene (LDPE) film surfaces by SF, and SF,-CF, plasma treatments. The influence of the plasma treatment time (SF, plasma) and the gas composition (SF6-CF, plasma) have been investigated. The ToF-SIMS results are compared with plasma diagnostics (mass spectrometry and optical emission spectroscopy) and with some earlier XPS characterization.The ToF-SIMS study of SF, plasma-treated LDPE films as a function of the treatment time shows two different structural developments. Beside the expected formation of C,F, structures due to the grafting of F atoms, ToFSIMS detects the presence of SF, species at the surface. Their high sensitivity to ablation, due to the plasma ions, was related to two possible interactions of plasma species with the LDPE surface: an adsorption of SF:/-ions or a dissociative chemisorption of SF, neutral species. In the latter case, the low binding energy of the S-C bond can explain the high sensitivity to ablation.In the case of SF,-CF, plasma treatment, the incorporation of fluorine, as seen by ToF-SIMS, is a function of the gas composition. Two maxima are observed when the second gas (either SF, or CF,) is introduced at low concentration (-10%). These maxima are confirmed by XPS and actinometric results and are explained with respect to the reciprocal influence of both gases in the plasma discharge.