Two soluble side-chain conjugated polythiophenes, poly {3-[2-(4-octyloxyphenyl)-vinyl]-thiophene} (P3OPVT) and poly{3-(4-octyloxy-phenylethynyl)-thiophene} (P3OPET) have been synthesized successfully. In P3OPVT and P3OPET, substituted benzene rings are connected with the polythiophene backbone through trans carbon-carbon double bond and carbon-carbon triple bond, respectively. Absorption spectra of the P3OPVT and P3OPET both show two absorption peaks located in UV and visible region, respectively. The results of optical and electrochemical measurements indicate that the conjugated side-chains can reduce the bandgap effectively. This type of side-chain conjugated polythiophenes may be promising for the applications in polymer photovoltaic cells and field effect transistors. (c) 2006 Wiley Periodicals, Inc.