Molecules capable of mediating charge transport over several nanometers with minimal decay in conductance have fundamental and technological implications. Polymethine cyanine dyes are fascinating molecular wires because up to a critical length, they have no bond-length alternation (BLA) and their electronic structure resembles a one-dimensional free-electron gas. Beyond this threshold, they undergo a symmetry-breaking Peierls transition, which increases the HOMO−LUMO gap. We have investigated cationic cyanines with central polymethine chains of 5−13 carbon atoms (Cy3 + −Cy11 + ). The absorption spectra and crystal structures show that symmetry breaking is sensitive to the polarity of the medium and the size of the counterion. X-ray crystallography reveals that Cy9•PF 6 and Cy11•B(C 6 F 5 ) 4 are Peierls distorted, with high BLA at one end of the π-system, away from the partially delocalized positive charge. This pattern of BLA distribution resembles that of solitons in polyacetylene. The single-molecule conductance is essentially independent of molecular length for the polymethine salts of Cy3 + − Cy11 + with the large B(C 6 F 5 ) 4− counterion, but with the PF 6 − counterion, the conductance decreases for the longer molecules, Cy7 + −Cy11 + , because this smaller anion polarizes the π-system, inducing a symmetry-breaking transition. At higher bias (0.9 V), the conductance of the shorter chains, Cy3 + −Cy7 + , increases with length (negative attenuation factor, β = −1.6 nm −1 ), but the conductance still drops in Cy9 + and Cy11 + with the small polarizing PF 6 − counteranion.