Carboxylate groups were introduced at poly(ethylene) (PE) surfaces by immobilization of sodium 10-undecenoate (C11(:)) or sodium dodecanoate (C12) using an argon plasma treatment. The presence of carboxylate groups at the surface was confirmed by ATR-FTIR spectroscopy, contact-angle determinations, and derivatization XPS. The presence of an unsaturated bond in the alkyl chain of the surfactant enhanced the immobilization process. About 28% of the initial amount of carboxylate groups in the precoated C11(:) layer was retained at the PE surface after plasma treatment, as compared to only 6% for the C12 layer. No changes in the wettability of the modified PE surfaces were observed when they were stored at room temperature for at least 12 weeks. The use of the carboxylate groups that were introduced at polymeric surfaces with this method for further covalent coupling of other molecules is discussed.