Naphthalene has been polymerized electrochemically in the mixed electrolyte of boron trifluoride diethyl etherate (BFEE) and trifluoroacetic acid (TFA). The oxidation potential of the monomer in this medium was measured as approximately 1.0 V (vs. SCE), which was much lower than that detected in a neutral electrolyte such as acetonitrile (1.55 V vs. SCE). A powdery polymer with a conductivity of 2.6×10−3 S cm−1 was obtained. The addition of a small amount (∼5%, by weight) of polyethylene glycol with a molar mass of 400 g (PEG 400) into the mixed electrolyte can decrease the oxidation potential of the monomer further to approximately 0.7 V versus SCE. A free-standing polynaphthalene (PNA) film with a conductivity of 5.9×10−2 S cm−1 was obtained from this medium for the first time. The structure of the polymer has been investigated by UV–Vis, infrared and Raman spectroscopies.