A new synthetic route to the low bandgap polymer poly(isothianaphthene) (PITN) is presented. This new route comprises the reaction of 1,1,3,3-tetrachlorothiophthalan with tert-butylmercaptan and trifluoroacetic acid as a catalyst. This reaction clarifies the mechanism proposed for the synthesis of PITN from phthalic anhydride and phosphorus pentasulfide. The obtained product was analysed by elemental analysis, FTi.r., FT Raman and solid state n.m.r, spectroscopy. Electrical conductivity after synthesis equals approximately 10(-2) S cm(-1). All experimental data are consistent with those of PITN obtained via other synthetic procedures. An indication of the number average molecular weight of PITN is given by quantitative end group analysing using CP/MAS n.m.r.