Optical absorption spectra of anions and cations of poly(3-decylthiophene) (P3DT) in solution were identified as single polarons. Pulse radiolysis of P3DT in THF determined the spatial extent of one negative polaron to be similar to 11.5 thiophene units by observing transient absorption of P3DT(-center dot) radical ions, which are prinicpally free ions, at 850 nm with epsilon = (7.25 +/- 0.47) x 10(4) M-1 cm(-1) and bleaching of the neutral absorption band at 450 nm. P3DT(-center dot) was formed in a combination of diffusive reactions and fast "step" processes. Similarly, a positive polaron of P3DT was estimated to delocalize over similar to 8.7 thiophene units by pulse radiolysis in chloroform. Chemical reduction by sodium and oxidation by FeCl3 injected multiple charges into a single P3DT chain while showing absorption spectra in the early stages of reaction resembling those observed by pulse radiolysis. The results indicated that multiple polarons exist in a single chain of P3DT before coalescing into bipolarons or transforming into other forms of polaron.