The deposition and structure of (protoporphyrinato(IX))iron(III) chloride (abbreviated as PP(IX)Fe(III)Cl) films on highly oriented pyrolytic graphite (HOPG) have been investigated by cyclic voltammetry, scanning tunneling microscopy (STM), and electrochemical quartz crystal microbalance (EQCM) measurements. PP(IX)Fe(III)Cl films are prepared by two methods : (1) adsorption, yielding an electrochemically active film; (2) electrooxidative polymerization, yielding a partially inert film. STM images, in conjunction with electrochemical results, indicate that adsorption of PP(IX)Fe(III)Cl from aqueous solutions onto HOPG results in a thin film (equivalent thickness of similar to 1-2 monolayers) that is comprised of 30-Angstrom-diameter molecular aggregates. In contrast, thicker films (similar to 50 nm) prepared by electrooxidative polymerization have a dense morphology, consistent with polymerization occurring by radical-cation-initiated coupling of the vinyl groups on the protoporphyrin ring. The number of electrons transferred per molecule deposited during electrooxidative polymerization has been measured by combined EQCM and coulometric measurements and is shown to be a strong function of potential, with a maximum value of n similar to 8. These results are consistent with the previously reported conclusion of Macer and Spiro that radical-cation polymerization is accompanied by metal-centered oxidation and the six-electron oxidation of the protoporphyrin ring to the electrochemically inert dioxoporphomethene species.