Thianaphthene has been electrochemically polymerized in pure boron trifluoride diethyl etherate (BFEE) solution or in a mixed electrolyte of BFEE and concentrated sulfuric acid (SA). The addition of a certain amount of sulfuric acid into BFEE accelerated the polymerization and also increased the current efficiency of the electrosynthesis. Poly(thianaphthene) (PTN) in the dedoped state is soluble in usual strong polar organic solvents such as dimethyl sulfoxide (DMSO), NN-dimethylformamide (DMF) and N-methyl pyrrolidinone (NMP). Its structure has been examined by infrared, H-1-NMR and UV spectra. Fluorescent spectral studies indicate that the polymer is a strong blue light emitter.