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The surfaces of untreated and helium plasma-based ion implantation (He PBII) treated poly(ethylene terephthalate) (PET) samples were characterised by reflectance colorimetry, contact angle studies and measurements of surface electrical resistance. The results were related to the structural and compositional data obtained by the authors earlier on parallel samples by XPS and Raman spectroscopy. Inverse correlations between lightness and I D /I G ratio and between chroma and I D /I G ratio were obtained, suggesting that the PBII-treated PET samples darken and their colourfulness decreases with the increase of the portion of aromatic sp 2 carbon rings in the chemical structure of the modified layer. Direct correlation between water contact angle and the I D /I G ratio and inverse correlations between surface energy and I D /I G ratio and between dispersive component of surface energy and I D /I G ratio were found, reflecting that surface wettability, surface energy and its dispersive component decrease with the formation of surface structure, characterised again by enhanced portion of aromatic sp 2 carbon rings. The surface electrical resistance decreased with the increase of the surface Ccontent determined by XPS and also with the increase of the surface concentration of conjugated double bonds, reflected by the increase of the * shake-up satellite of the C 1s peak.Keywords: poly(ethylene terephthalate), plasma-based ion implantation, colorimetry, wettability, electrical resistance IntroductionPlasma-based ion implantation (PBII), called also plasma immersion ion implantation (PIII or PI 3 ) is a versatile, relatively simple and cost-effective surface modification technique, alternative to conventional ion implantation. PBII treatment of poly(ethylene terephthalate) (PET) is a subject of current interest, due to promising applications in areas including packaging and biomaterial devices [1,2 and references therein]. Recently we studied the surface modification of PET by He PBII, applying XPS and Raman spectroscopy, including the effect of the main process parameters (acceleration voltage, fluence and fluence rate) on the alterations of the surface chemical composition and structure [2]. An important feature of using helium for PBII treatment is that in the energy range of interest of PBII treatment (actually above 2 keV), the stopping power of PET for He + ions is dominated by the electronic component [2], and therefore the structure of the modified layer may become ordered [3]. Due to its small atomic size, He + ion has a relatively large projected range, as