Thin films of poly(3,4-ethylenedioxythiophene) (PEDOT) were fabricated via the vapor-phase polymerization (VPP) of the monomer 3,4-ethylenedioxythiophene (EDOT) with FeCl3·6H2O as the oxidant. The doping level and electrical properties of the VPP-PEDOT films were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrical conductivity measurements. The VPP of EDOT was observed by in situ ultraviolet–visible (UV–Vis) absorption spectroscopy. The results showed that highly doped PEDOT films were formed in the initial stages of VPP, and the doping level increased as the reaction proceeded. The doping level eventually decreased because of the limited amount of the oxidant. Therefore, it was concluded that the doping level of VPP-PEDOT thin films could be changed depending on the initial amount and supply of the oxidant. The thermoelectric properties of the as-synthesized VPP-PEDOT films were also studied, with the aim of applying them as an energy harvester and/or a temperature sensor.