Ion-implantation-induced electrical conductivity in a polymer surface is known to have a different mechanism from that of metals and semiconductors. We used a technique called plasma immersion ion implantation and deposition and combined it with a titanium cathodic vacuum arc to modify the surface electrical conductivity of poly(ethylene terephthalate) (PET). The conductivity curve as a function of temperature well fitted the Mott hopping model, which has been proposed for many disordered systems. In addition, we also observed conductivity degradation when modified PET was kept at room temperature. The degradation showed a quasi-exponential decay as a function of time, that is, an aging effect, which has been seldom reported in the literature to the best of our knowledge. This could have resulted from the unusual structure of PET's surface after ion implantation. A new formula for electrical conductivity in modified PET is proposed that considers both temperature and aging effects. (c) 2007 Wiley Periodicals, Inc.