Soluble polymers (PQE and PSE) with each constant rr-conjugation length, containing thiophene tetramers and hexamers in the main chains, were prepared by reductive condensation, respectively. The obtained polymers were soluble in common organic solvents. Cast films could be obtained from their solutions. The PQE and PSE films doped with FeCl3 showed considerable conductivities, 9-7 x 10(-3) and 0.4-0.2 Stem, respectively. The polymer solutions and films were oxidized (doped) by stepwise addition of FeCl3 and characterized by electronic absorption spectra. The spectral change of the PQE solutions and films showed one-step oxidation to produce cation radicals, whereas that of the PSE solutions and films did two-step oxidation to yield cation radicals in the fil:st step and plural-electron-oxidized species such as dications in the second step. In atmosphere, the plural-electron-oxidized species of the PSE films are extremely labile. The spectra of the oxidized PSE films in atmosphere were almost similar to that of doped polythiophene films, indicating that oxidized species existing in doped polythiophenes are mainly cation radicals.