The surfaces of poly(styrene-co-acrylic acid) copolymers and their Na- and Cs-neutralized ionomers were modified by O-2-plasma source ion implantation (PSII) treatment to improve the surface wettability. The changes in the surface wettability, composition, and structure upon the PSII treatment were examined with contact-angle measurements and X-ray photoelectron spectroscopy. The untreated surfaces of the acid copolymers and ionomers exhibited different surface energies; this implied clearly that the type of ion species affects the surface hydrophilicity. Also, the PSII treatment induced oxygen-containing groups to reside on the surface and ionic groups to come out toward the surface; this made the surfaces of the ionomers more hydrophilic as compared with that of the acid copolymers. The ionomers also showed slow hydrophobic recovery. Thus, it was suggested that the reduced mobility of the polymer chain because of the presence of ionic aggregates results in restricted reorientation of oxygen-containing groups. (C) 2003 Wiley Periodicals, Inc.