The polymerization of 3-undecylbithiophene in supercritical carbon dioxide (with ferric triflate as the oxidant) was studied at 313 K and 10.5 MPa. The resulting polymer was comparable in structure, molecular weight, conjugation, and intrinsic electrical conductivity to the polymer synthesized in nitrobenzene (with ferric chloride as the oxidant). In addition, a 95% yield of the conducting polymer was obtained in 1 h. Conductive composites of poly(3-undecylbithiophene) and porous, cross-linked polystyrene were prepared via the in situ polymerization of 3-undecylbithiophene in supercritical carbon dioxide at temperatures in the range 313-413 K and pressures in the range 10.5-34.5 MPa. The highest conductivities of the composite were obtained at 313 K and 20.7 MPa, which represent optimum conditions for oxidant/monomer deposition and conducting polymer network formation on the surface and pores of the host polymer.