The de conductivity of poly(3-alkyl thiophenes) (P3ATs) and their cocrystals are measured in the temperature range of -130 to 150°C. Both solvent-cast films and the melt-cooled films are used. The former exhibit a sharp increase followed by a decrease in conductivity, whereas the latter show only a sigmoidal increase in conductivity, with temperature. The sigmoidal increase of the melt-cooled samples is dependent on the regioregularity and alkyl chain length of the samples and is explained from the solid-state transformation of the interdigited type-II crystal to a noninterdigited type-I crystal. The type-I crystal itself has lower conductivity than type-II crystals, and the samples exhibit a blue shift in UV-vis absorption spectra with an increase in temperature. So the sigmoidal increase of conductivity is attributed to the increase in carrier mobility with temperature in the type-I crystals. The X-ray and DSC results suggest that during the transformation of type-II to type-I crystals with increasing temperature, localized crystallites of smaller dimensions separated by narrower amorphous portions are produced throughout the whole matrix. But in the type-II polymorph, the crystallites are large with a wider gap in between. The sigmoidal increase is attributed to the easier hopping of the charge carrier among the localized crystallites of the type-I crystal with increasing temperature. In the cocrystals, the smaller sigmoidal increase with increasing irregular sample concentration is valid for a lower concentration of such localized crystallites. The same is true for cocrystals with longer alkyl chain lengths. The interfibrillar contribution of conductivity through the network junctions together with the carrier hopping between the localized crystallites might be a possible reason for the metallic-type behavior of solvent-cast samples after a certain temperature. The melt-cooled films do not exhibit metallic-type behavior because of the absence of a fibrillar network morphology. © 2005 American Chemical Society.