The "doping" of trans-1,4-polybutadiene in the solid state with iodine has been reported to convert this polymer into a semiconductor at room temperature, but an analogous conversion is not observed on iodine doping of cis-1,4-polybutadiene. In this study, the reactions of 1,4-polybutadienes with iodine have been investigated in solution. UV/vis, IR, and NMR spectroscopic measurements, photosensitized cis-trans isomerization studies, and molecular orbital calculations indicate that the "I(2)doping" of trans-1,4-polybutadiene results in the formation of sequences of conjugated double bonds, which confer conductivity. In contrast, the I-2 doping of cis-1,4-polybutadiene does not lead to the formation of conjugated sequences at room temperature, due to an unfavorable combination of electronic and steric interactions associated with the polymer backbone geometry. The marked difference between the results of I-2 doping of cis-l,4-polybutadiene and those of trans-1,4-polybutadiene is thus interpreted in terms of differences in their intramolecular structure, rather than differences in intermolecular packing as previously proposed by others.