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The mechanism of the electrochemical polymerization of pyrrole and pyrrole‐d4 has been studied by an examination of the kinetic deuterium isotope effect. The kinetic rate constants of the reaction were measured by cyclic voltammetry, UV/visible specrophotometric analysis of polymer growth, and quantitative gas chromatographic analysis of monomer uptake. All three techniques demonstrated no primary isotope effect, from which it is concluded that proton loss or abstraction is not involved in the rate‐determining step of the polymerization. Furthermore no consistent secondary effect could be observed, suggesting that the rate‐determining step is free of hybridization changes or hyperconjugative effects in the transition state. A radical trap experiment revealed that polymer formation is completely inhibited by the presence of a radical trap and thus a reaction with the scavenger is more favorable than coupling with another radical cation or a neutral monomer molecule. Previously proposed mechanisms are examined and compared in light of the present study. Oxidative coupling is favored as the mechanism for electrochemical polymerization of pyrrole and related compounds. For this mechanism the rate‐limiting step must be either oxidation of monomer or radical cation coupling.

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