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Electrooxidation in organic solvents of the dimethyl esters of several metalloprotoporphyrins (PP) (NinPP, ZnnPP, ConPP, (FemPP)Cl, (FemPP)20, and (CrmPP)20) leads to the deposition of thick (~1 000 monolayer equivalents), electroactive porphyrin films, which have been characterized by cyclic voltammetry and absorption spectroscopy on transparent Sn02 electrodes.The films are stable toward organic solvents and aqueous acids and bases, but are removed by treatment with hot concentrated acids. The resonance Raman spectrum of the NiPP film indicates that one of the two vinyl groups is saturated on most of the porphyrin units. Deposition continues for some minutes after the current is interrupted. This evidence is consistent with a mechanism involving electroinitiated cationic vinyl polymerization. No film is formed if the metal, rather than the ring, is oxidized. Thus the first oxidation step of ConPP, to [ComPP]+, does not support film formation (although the potential is as high as for ring oxidation in ZnPP), but the second step, to [Co111??•]2"1", does. Lack of film formation for (MnnlPP)Cl and (CrIVPP)0 suggests metal, rather than ring oxidation, to MnIV and Crv. However, (CrPP)20 oxidation does produce a film, suggesting ring oxidation, analogous to (FePP)20, which also produces a film. However, while (CrPP)20 is incorporated intact into the film, the (FePP)20 film contains monomer units. Incorporation of other metal ions can be accomplished by soaking a ZnPP-coated electrode in H2S04 followed by contact with a solution of the metal dihalide in refluxing DMF. The porphyrin sites are accessible to small ions, as shown by chloride coordination of ZnPP film upon soaking in chloride solution.