The mechanical stability of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films spin coated onto a UV treated PET substrate was investigated. It was found that the UV exposure modifies the PET surface and improves the wetting conditions for PEDOT:PSS films deposition. The adhesion strength of PEDOT:PSS to UV treated PET surface was found to be 50 N/cm2 versus 19 N/cm2 to the untreated one. Furthermore, the resistance change of PEDOT:PSS on UV treated PET at 20,000 cycles of bending was measured to be only 3.8%. In contrast, the same polymeric films on untreated PET exhibit 8.7% resistance change and the conventional indium tin oxide (ITO) electrode 23.5%, which demonstrates the superior flexibility of the UV PET/PEDOT:PSS electrodes. The transparency of the produced coatings is 81%. Single layer organic light-emitting device with MEH-PPV electroluminescent polymer was successfully fabricated on UV PET/PEDOT:PSS. The electrical measurements indicate the possibility of using UV PET/PEDOT:PSS as a transparent electrode system, replacing ITO in the flexible optoelectronic devices.