Self-assembled monolayers (SAMs) of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) and cobalt 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (CoTPyP) were formed by in situ equilibrium adsorption onto Au(1 1 1) electrodes passivated by an interposed adlayer of iodine. Adsorption of the porphyrins onto Au(1 1 1) was confirmed by cyclic voltammetry, and each of the SAMs was imaged using electrochemical scanning tunneling microscopy (EC-STM). The TPyP and CoTPyP SAMs were found to contain highly ordered domains on I-Au( 1 1 1) at positive potentials where a disordered SAM would be expected for bare Au(1 1 1). Cyclic voltammetry was also used to observe in situ the assembly of binary SAMs containing TPyP and CoTPyP as they formed. (C) 2011 Elsevier B.V. All rights reserved.