A solution of processible poly(3-pentylthiophene) (P3PT) was synthesized by chemical polymerization. The structure of the resulting polymer was investigated by gel permeation chromatography (GPC); infrared; H-1 and C-13 nuclear magnetic resonance; Ultraviolet visible spectroscopy (UV-Vis) and wide-angle X-ray scattering (WAXS) techniques. It was found that the microstructure of the chains has a low number of irregular couplings and thus has desirable electrical properties. In the small-angle region of X-ray scattering, a pronounced peak was observed with a d (coherence length) value of 1.51 nm for P3PT and 1.835 nm for poly (3-heptylthiophene (P3Hept). Electron spin resonance measurements showed that P3PT has 10(18) spins g-1, a g value 2.0039, and a DELTAH(pp) of about 6 Gauss. The Pauli-like susceptibility after doping P3PT with I2 is of the same order as for poly paraphenylene (PPP) doped with AsF5, and corresponds to a density of states at the Fermi level about 0.01 states eV-1 per carbon atom in the thiophene ring. The current-voltage (I-V) characteristics obtained for different temperatures were always linear. The dark room temperature conductivity was 10(-8) S m-1 and distinctly increased.after doping. The P3PT studied is a good candidate for potential commercial applications.