Electron paramagnetic resonance (EPR) and electrical conductivity are studied in undoped, iodine- and Cu(II) complex-doped poly(isothianaphthene) (PITN) powders prepared by O₂-induced polymerization of 1,3-dihydroisothianaphthene. The integrated intensity of the EPR signals of iodine-doped PITN increases as the dopant concentration increases up to 5%, then decreases, while the conductivity shows a sharp increase at a doping level of about 9%. The EPR linewidth decreases slightly upon dedoping and increases as the doping level increases. These results are discussed in terms of the formation of polarons and bipolarons and a transition from a semiconductor to a metallic regime upon doping. PITN samples doped with three different anionic Cu(II) complexes by an ion-exchange process show a Cu(II) doping level that is 3–5 times greater than that in samples prepared by a direct doping process. Cu(II) complex-doped PITN (σ_max = 5.2 S cm⁻¹) gives better conductivity than iodine-doped PITN (σ_max =  0.16 S cm⁻¹) even at lower doping levels. This is possibly due to an increase in the interchain contribution to the conduction mechanism.