We have used laser desorption Fourier transform mass spectrometry to determine the molecular weight distributions of three forms of poly(2-vinylthiophene) : polymer A, the classic, free radical-initiated form, polymer B, produced as a precipitate during the anodic oxidation of the monomer, and polymer C, formed as an insoluble film on the anode during the same oxidation. We have obtained molecular weight distributions for all three of these products in the form of Na+-attached species. Analysis of the mass spectral data allows us to reach several conclusions. For polymer A, initiation takes place by benzoate radicals, and this reaction is faster than decomposition of benzoate radicals to phenyl radicals and carbon dioxide. Propagation takes place by the well-established radical mechanism, and termination occurs mainly by chain transfer to monomer with some disproportionation. For polymer B, two mechanisms are consistent with the mass spectral data. The first involves free radical chain propagation involving loss of a proton after the initiation step. Termination can occur by combination, disproportionation, or chain transfer. The second mechanism involves combination of two initially formed radical cations with subsequent cationic propagation. Termination involves the loss of two protons. The second mechanism alone does not adequately explain the data. For polymer C, infrared spectral data support a mechanism that involves initial ring-ring radical coupling followed by loss of two protons and further oxidation of the resulting dimer, leading to a polymer containing some cross-linking. The mass spectral data are consistent with this interpretation.