Surfaces of poly(ethylene terephthalate); PET, films were irradiated with Ar+ at 1 keV using various ion doses (ID) from 10(14) to 10(17) ions/cm(2) (isc) with and without an O-2 environment. The wettability of the modified surfaces of PET was determined by measuring the contact angle between water droplets and the modified surfaces. The modified surfaces were also characterized by AFM (atomic force microscopy) and XPS (X-ray photoelectron spectroscopy) for changes in the surface morphology, and the chemical composition and molecular structure, respectively. The contact angle decreased from 70 degrees for unmodified surfaces to 45 degrees for modified surface with ID = 10(14) isc without O-2 and remained relatively constant with higher ID. The contact angle, however, reached a minimum value of 8 degrees for modified surfaces with ID = 10(16) isc with O-2. The improved wettability may be due to a combination of the formation of hydrophilic groups, chemical and molecular structural changes, physical structural or morphological changes, and increased roughness of the surface. The wettability of the modified surfaces also depended on the time of exposure to air. The wettability worsened with exposure time to air, but was revived by immersing the films into water. Possible mechanisms for the change of the wettability of the modified surfaces are given.