Using the rotating ring-disk electrode we have observed what appears to be the primary oxidation product, a pyrrole species, in the electro-oxidative polymerization of pyrrole to form conducting polypyrrole. An E(o)’ for this species of + 1.1 V vs. the saturated sodium chloride calomel electrode (SSCE) in 0.1 M NaClO4 acetonitrile solution is estimated, in agreement with recent cyclic voltammetric observations by others at ultra-high sweep rates. A crude kinetic calculation of the ring current-disk potential curve for the radical gives an estimated overall rate constant of 10(7)-10(8) 1 Mol-1 s-1, indicating near diffusion control. At constant disk potential an induction period before the onset of significant pyrrole oxidation was observed, and the rapid rise in current coincides with a substantial, transient increase in the production of pyrrole radical species. During this rapid current increase polymer is deposited non-uniformly on the disk electrode in a pattern that shows that the hydrodynamic flow profile at the rotating electrode controls polymer growth. The pattern indicates that soluble, reactive pyrrole oxidation products play a key role in the growth of the film. The process is similar to three-dimensional nucleation and growth during metal electrodeposition, but differs in the role of soluble intermediates.