Current understanding of the structure, spectra, and conduc-tivity of oxidized polythiophene and other conducting polymers is dominated by the theory that polarons and bipolarons are formed along single conjugated chains. 1 Because polarons andbipolarons have small-molecule analogues in cation radicals and dications, it is natural to explore that theory by generating these cations and studying their properties. 2 We report that the cation radicals of terthiophenes reversibly dimerize even at low concentration. We suggest that these are-dimers and that such dimers are reasonable alternatives to bipolarons. As a consequence,-dimers and-stacks deserve attention as entities responsible for the properties of oxidized polythiophene and other conducting poly-mers.(1)(a) Ferraro, J. R.; Williams, J. M. Introduction to Synthetic Electrical Conductors·, Academic Press: NewYork, 1987.(b) Handbook of Conducting Polymers·, Skotheim, TA, Ed.; Dekker: New York, 1986; Vols. 1 and 2.(2)(a) Chang, A.-C; Miller, LL Synth. Met. 1987, 22, 71.(b) Fichou, D.; Horowitz, G.; Xu, B.; Gamier, F. Synth. Met. 1990, 39, 243.(c) Fichou, D.; Horowitz, G. Mater. Res. Soc. Symp. Proc. 1990, 173, 379.(d) Caspar, J. V.; Ramamurthy, V.; Corbin, DRJ Am. Chem. Soc. 1991, 113, 600.