Dichloromethane and acetonitrile containing thiophene and oxidant, respectively, were adopted as a binary organic solvent for oxidative synthesis of unsubstituted polythiophene (UPTh) under various conditions, i.e., oxidant/monomer molar ratio, polymerization temperature, and reaction time. We also investigated the effects of polymerization medium on the chemical and physical properties of the UPTh. The electrical conductivities of the UPThs polymerized in only acetonitrile or aqueous medium were 1.0 x 10(-4) Scm(-1) and zero, respectively. However, the UPTh obtained in binary organic solvent system exhibited the highest electrical conductivity of 20.1 Scm(-1) without post-doping process under an oxidant/monomer molar ratio of 5:1 and a reaction time of 0.2 h at 0 degrees C. Furthermore, it showed an enhanced crystalline structure, thermal stability, and longer pi-conjugation length than those in acetonitrile or aqueous medium. We also found that the presence of water as a polymerization medium inevitably gave rise to the incorporation of hydroxyl and carbonyl groups into the UPTh main chains by nucleophilic attack of water molecules. Therefore, these undesired structures broke the conjugated structures, resulting in UPTh with an electrically non-conducting property. The morphology of UPTh prepared in binary organic solvent appeared to be an aggregate of nanoparticles (50-200 nm). The combination of organic solvents should provide another systematic approach to the facile and rapid synthesis of UPThs with high conductivity. (C) 2010 Elsevier Ltd. All rights reserved.