We have studied the adsorption and electropolymerization of protoporphyrin IX (PP), Fe(III)-protoporphyrin IX (Fe-PP) and Zn(II)-protoporphyrin IX (Zn-PP) on highly oriented pyrolytic graphite (HOPG) with in situ atomic force microscopy (AFM) and scanning tunneling microscopy (STM) and examined the performance of the porphyrin-coated electrodes for electrochemical detection of the neurotransmitter, dopamine. AFM and STM reveal that the porphyrins adsorb spontaneously onto HOPG and self-assemble into ordered monolayers with the individual molecules lying flat on the surface. On increasing the electrode potential to the oxidation potentials, islands, corresponding to electrooxidative polymerization, begin to form on the monolayers. The islands grow in size and number and form a continuous film that provides a convenient way to modify the graphite electrode. The porphyrin-coated electrodes can suppress the unwanted oxidation of ascorbic acid by 100-fold without sacrificing much of the electrode response time and sensitivity for detecting dopamine.