The oxidation of electrochemically grown poly(3-methylthiophene) and its other spectroscopic properties have been studied by in-situ spectroelectrochemlcal techniques as well as in-situ conductivity measurements, and the results are reported. An absorptivity of 1.1 X 105 cm'1 is reported for the absorption band at 490 nm for a neutral, reduced polymer film grown in propylene carbonate. The oxidation of the neutral polymer to the cation radical, or polaron, and its further oxidation to the dication, or bipolaron, are shown to take place at about 0.45 and 0.80 V, respectively, and be controlled by the diffusion of counterions. In-situ conductivity measurements of the film show that both the polaron and bipolaron are charge carriers. We conclude from the results that at least two chemically and optically different species, radical cation and dication, and perhaps other products, are produced in different potential regimes upon oxidation of poly(3-methylthiophene).